[423]*423Justice Stevens
delivered the opinion of the Court.
Since 1980, the Immigration and Nationality Act has provided two methods through which an otherwise deportable alien who claims that he will be persecuted if deported can seek relief. Section 243(h) of the Act, 8 U. S. C. § 1253(h), requires the Attorney General to withhold deportation of an alien who demonstrates that his “life or freedom would be threatened” on account of one of the listed factors if he is deported. In INS v. Stevic, 467 U. S. 407 (1984), we held that to qualify for this entitlement to withholding of deportation, an alien must demonstrate that “it is more likely than not that the alien would be subject to persecution” in the country to which he would be returned. Id., at 429-430. The Refugee Act of 1980, 94 Stat. 102, also established a second type of broader relief. Section 208(a) of the Act, 8 U. S. C. § 1158(a), authorizes the Attorney General, in his discretion, to grant asylum to an alien who is unable or unwilling to return to his home country “because of persecution or a well-founded fear of persecution on account of race, religion, nationality, membership in a particular social group, or political opinion.” § 101(a)(42), 8 U. S. C. § 1101(a)(42).
In Stevie, we rejected an alien’s contention that the § 208(a) “well-founded fear” standard governs applications for withholding of deportation under 1243(h).1 Similarly, today we reject the Government’s contention that the § 243(h) standard, which requires an alien to show that he is more likely than not to be subject to persecution, governs applications for asylum under § 208(a). Congress used different, broader language to define the term “refugee” as used in § 208(a) than it used to describe the class of aliens who have [424]*424a right to withholding of deportation under § 243(h). The Act’s establishment of a broad class of refugees who are eligible for a discretionary grant of asylum, and a narrower class of aliens who are given a statutory right not to be deported to the country where they are in danger, mirrors the provisions of the United Nations Protocol Relating to the Status of Refugees, which provided the motivation for the enactment of the Refugee Act of 1980. In addition, the legislative history of the 1980 Act makes it perfectly clear that Congress did not intend the class of aliens who qualify as refugees to be coextensive with the class who qualify for § 243(h) relief.
I
Respondent is a 38-year-old Nicaraguan citizen who entered the United States in 1979 as a visitor. After she remained in the United States longer than permitted, and failed to take advantage of the Immigration and Naturalization Service’s (INS) offer of voluntary departure, the INS commenced deportation proceedings against her. Respondent conceded that she was in the country illegally, but requested withholding of deportation pursuant to § 243(h) and asylum as a refugee pursuant to § 208(a).
To support her request under § 243(h), respondent attempted to show that if she were returned to Nicaragua her “life or freedom would be threatened” on account of her political views; to support her request under § 208(a), she attempted to show that she had a “well-founded fear of persecution” upon her return. The evidence supporting both claims related primarily to the activities of respondent’s brother who had been tortured and imprisoned because of his political activities in Nicaragua. Both respondent and her brother testified that they believed the Sandinistas knew that the two of them had fled Nicaragua together and that even though she had not been active politically herself, she would be interrogated about her brother’s whereabouts and [425]*425activities. Respondent also testified that because of her brother’s status, her own political opposition to the Sandinis-tas would be brought to that government’s attention. Based on these facts, respondent claimed that she would be tortured if forced to return.
The Immigration Judge applied the same standard in evaluating respondent’s claim for withholding of deportation under § 248(h) as he did in evaluating her application for asylum under § 208(a). He found that she had not established “a clear probability of persecution” and therefore was not entitled to either form of relief. App. to Pet. for Cert. 27a. On appeal, the Board of Immigration Appeals (BIA) agreed that respondent had “failed to establish that she would suffer persecution within the meaning of section 208(a) or 243(h) of the Immigration and Nationality Act.” Id., at 21a.
In the Court of Appeals for the Ninth Circuit, respondent did not challenge the BIA’s decision that she was not entitled to withholding of deportation under § 243(h), but argued that she was eligible for consideration for asylum under § 208(a), and contended that the Immigration Judge and BIA erred in applying the “more likely than not” standard of proof from § 243(h) to her § 208(a) asylum claim. Instead, she asserted, they should have applied the “well-founded fear” standard, which she considered to be more generous. The court agreed. Relying on both the text and the structure of the Act, the court held that the “well-founded fear” standard which governs asylum proceedings is different, and in fact more generous, than the “clear probability” standard which governs withholding of deportation proceedings. 767 F. 2d 1448, 1452-1453 (1985). Agreeing with the Court of Appeals for the Seventh Circuit, the court interpreted the standard to require asylum applicants to present “ ‘specific facts’ through objective evidence to prove either past persecution or ‘good reason’ to fear future persecution.” Id., at 1453 (citing Carvajal-Munoz v. INS, 743 F. 2d 562, 574 (CA7 1984)). [426]*426The court remanded respondent’s asylum claim to the BIA to evaluate under the proper legal standard. We granted cer-tiorari to resolve a Circuit conflict on this important question.2 475 U. S. 1009 (1986).3
[427]*427I — I HH
The Refugee Act of 1980 established a new statutory procedure for granting asylum to refugees.4 The 1980 Act added a new § 208(a) to the Immigration and Nationality Act of 1952, reading as follows:
“The Attorney General shall establish a procedure for an alien physically present in the United States or at a land border or port of entry, irrespective of such alien’s status, to apply for asylum, and the alien may be granted asylum in the discretion of the Attorney General if the Attorney General determines that such alien is a refugee within the meaning of section 1101(a)(42)(A) of this title.” 94 Stat. 105, 8 U. S. C. § 1158(a).
Under this section, eligibility for asylum depends entirely on the Attorney General’s determination that an alien is a [428]*428“refugee,” as that term is defined in § 101(a)(42), which was also added to the Act in 1980. That section provides:
“The term ‘refugee’ means (A) any person who is outside any country of such person’s nationality or, in the case of a person having no nationality, is outside any country in which such person last habitually resided, and who is unable or unwilling to return to, and is unable or unwilling to avail himself or herself of the protection of, that country because of persecution or a well-founded fear of persecution on account of race, religion, nationality, membership in a particular social group, or political opinion . . . 94 Stat. 102, 8 U. S. C. § 1101(a)(42).
Thus, the “persecution or well-founded fear of persecution” standard governs the Attorney General’s determination whether an alien is eligible for asylum.5
In addition to establishing a statutory asylum process, the 1980 Act amended the withholding of deportation provision,6 [429]*429§ 243(h). See Stevic, 467 U. S., at 421, n. 16. Prior to 1968, the Attorney General had discretion whether to grant withholding of deportation to aliens under § 243(h). In 1968, however, the United States agreed to comply with the substantive provisions of Articles 2 through 34 of the 1951 United Nations Convention Relating to the Status of Refugees. See 19 U.S.T. 6223, 6259-6276, T.I.A.S. No. 6577 (1968); see generally Stevie, supra, at 416-417. Article 33.1 of the Convention, 189 U.N.T.S. 150, 176 (1954), reprinted in 19 U.S.T. 6259, 6276, which is the counterpart of §243(h) of our statute, imposed a mandatory duty on contracting States not to return an alien to a country where his “life or freedom would be threatened” on account of one of the enumerated reasons.7 See infra, at 441. Thus, although § 243(h) itself did not constrain the Attorney General’s discretion after 1968, presumably he honored the dictates of the United Nations Convention.8 In any event, the 1980 Act removed the Attorney General’s discretion in § 243(h) proceedings.9
[430]*430In Stevie we considered it significant that in enacting the 1980 Act Congress did not amend the standard of eligibility for relief under § 243(h). While the terms “refugee” and hence “well-founded fear” were made an integral part of the § 208(a) procedure, they continued to play no part in § 243(h). Thus we held that the prior consistent construction of § 243(h) that required an applicant for withholding of deportation to demonstrate a “clear probability of persecution” upon deportation remained in force. Of course, this reasoning, based in large part on the plain language of § 243(h), is of no avail here since § 208(a) expressly provides that the “well-founded fear” standard governs eligibility for asylum.
The Government argues, however, that even though the “well-founded fear” standard is applicable, there is no difference between it and the “would be threatened” test of § 243(h). It asks us to hold that the only way an applicant can demonstrate a “well-founded fear of persecution” is to prove a “clear probability of persecution.” The statutory language does not lend itself to this reading.
To begin with, the language Congress used to describe the two standards conveys very different meanings. The “would be threatened” language of § 243(h) has no subjective component, but instead requires the alien to establish by objective evidence that it is more likely than not that he or she will be subject to persecution upon deportation.10 See Stevie, supra. In contrast, the reference to “fear” in the § 208(a) standard obviously makes the eligibility determination turn to some extent on the subjective mental state of the [431]*431lien.11 “The linguistic difference between the words ‘well-ounded fear’ and ‘clear probability’ may be as striking as that jetween a subjective and an objective frame of reference.
. . We simply cannot conclude that the standards are identi:al.” Guevara-Flores v. INS, 786 F. 2d 1242, 1250 (CA5 1986), cert. pending, No. 86-388; see also Carcamo-Flores v. INS, 805 F. 2d 60, 64 (CA2 1986); 767 F. 2d, at 1452 (case below).
That the fear must be “well-founded” does not alter the obvious focus on the individual’s subjective beliefs, nor does it transform the standard into a “more likely than not” one. One can certainly have a well-founded fear of an event happening when there is less than a 50% chance of the occurrence taking place. As one leading authority has pointed out:
“Let us . . . presume that it is known that in the applicant’s country of origin every tenth adult male person is either put to death or sent to some remote labor camp. ... In such a case it would be only too apparent that anyone who has managed to escape from the country in question will have ‘well-founded fear of being persecuted’ upon his eventual return.” 1 A. Grahl-Madsen, The Status of Refugees in International Law 180 (1966).
This ordinary and obvious meaning of the phrase is not to be lightly discounted. See Russello v. United States, 464 U. S. 16, 21 (1983); Ernst & Ernst v. Hochfelder, 425 U. S. 185, 198-199 (1976). With regard to this very statutory scheme, we have considered ourselves bound to “ ‘assume “that .the legislative purpose is expressed by the ordinary meaning of the words used.”’” INS v. Phinpathya, 464 U. S. 183, 189 (1984) (quoting American Tobacco Co. v. Patterson, 456 [432]*432U. S. 63, 68 (1982), in turn quoting Richards v. United States, 369 U. S. 1, 9 (1962)).
The different emphasis of the two standards which is so clear on the face of the statute is significantly highlighted by the fact that the same Congress simultaneously drafted § 208(a) and amended § 243(h). In doing so, Congress chose to maintain the old standard in § 243(h), but to incorporate a different standard in § 208(a). “ ‘[W]here Congress includes particular language in one section of a statute but omits it in another section of the same Act, it is generally presumed that Congress acts intentionally and purposely in the disparate inclusion or exclusion.’” Russello v. United States, supra, at 23 (quoting United States v. Wong Kim Bo, 472 F. 2d 720, 722 (CA5 1972)). The contrast between the language used in the two standards, and the fact that Congress used a new standard to define the term “refugee,” certainly indicate that Congress intended the two standards to differ.
I — l I — I
The message conveyed by the plain language of the Act is confirmed by an examination of its history.12 Three aspects of that history are particularly compelling: The pre-1980 experience under § 203(a)(7), the only prior statute dealing with asylum; the abundant evidence of an intent to conform the definition of “refugee” and our asylum law to the United Nations Protocol to which the United States has been bound [433]*433since 1968; and the fact that Congress declined to enact the Senate version of the bill that would have made a refugee ineligible for asylum unless “his deportation or return would be prohibited by § 243(h).”
The Practice Under § 203(a)(7).
The statutory definition of the term “refugee” contained in § 101(a)(42) applies to two asylum provisions within the Immigration and Nationality Act.13 Section 207, 8 U. S. C. § 1157, governs the admission of refugees who seek admission from foreign countries. Section 208, 8 U. S. C. § 1158, sets out the process by which refugees currently in the United States may be granted asylum. Prior to the 1980 amendments there was no statutory basis for granting asylum to aliens who applied from within the United States.14 Asylum for aliens applying for admission from foreign countries had, however, been the subject of a previous statutory provision, and Congress’ intent with respect to the changes that it sought to create in that statute are instructive in discerning the meaning of the term “well-founded fear.”
Section § 203(a)(7) of the pre-1980 statute authorized the Attorney General to permit “conditional entry” to a certain number of refugees fleeing from Communist-dominated areas or the Middle East “because of persecution or fear of persecution on account of race, religion, or political opinion.” 79 [434]*434Stat. 913, 8 U. S. C. § 1153(a)(7) (1976 ed.). The standard that was applied to aliens seeking admission pursuant to § 203(a)(7) was unquestionably more lenient than the “clear probability” standard applied in § 243(h) proceedings. In Matter of Tan, 12 I. & N. Dec. 564, 569-570 (1967), for example, the BIA “found no support” for the argument that “an alien deportee is required to do no more than meet the standards applied under section 203(a)(7) of the Act when seeking relief under section 243(h).” Similarly, in Matter of Adamska, 12 I. & N. Dec. 201, 202 (1967), the Board held that an alien’s inability to satisfy § 243(h) was not determinative of her eligibility under the “substantially broader” standards of § 203(a)(7). One of the differences the Board highlighted between the statutes was that § 243(h) requires a showing that the applicant “would be” subject to persecution, while § 203(a)(7) only required a showing that the applicant was unwilling to return “because of persecution or fear of persecution.” 12 I. & N., at 202 (emphasis in original). In sum, it was repeatedly recognized that the standards were significantly different.15
At first glance one might conclude that this wide practice under the old § 203(a)(7), which spoke of “fear of persecution,” is not probative of the meaning of the term “well-founded fear of persecution” which Congress adopted in 1980. Analysis of the legislative history, however, demonstrates that Congress added the “well-founded” language only because that was the language incorporated by the United Nations Protocol to which Congress sought to conform. See infra, at 436-437. Congress was told that the extant asylum proce[435]*435dure for refugees outside of the United States was acceptable under the Protocol, except for the fact that it made various unacceptable geographic and political distinctions.16 The legislative history indicates that Congress in no way wished to modify the standard that had been used under § 203(a)(7).17 [436]*436Adoption of the INS’s argument that the term “well-founded fear” requires a showing of clear probability of persecution would clearly do violence to Congress’ intent that the standard for admission under §207 be no different than the one previously applied under § 203(a)(7).18
The United Nations Protocol.
If one thing is clear from the legislative history of the new definition of “refugee,” and indeed the entire 1980 Act, it is that one of Congress’ primary purposes was to bring United States refugee law into conformance with the 1967 United Nations Protocol Relating to the Status of Refugees, 19 U.S.T. 6223, T.I.A.S. No. 6577, to which the United States [437]*437acceded in 1968.19 Indeed, the definition of “refugee” that Congress adopted, see supra, at 428, is virtually identical to the one prescribed by Article 1(2) of the Convention which defines a “refugee” as an individual who
“owing to a well-founded fear of being persecuted for reasons of race, religion, nationality, membership of a particular social group or political opinion, is outside the country of his nationality and is unable or, owing to such fear, is unwilling to avail himself of the protection of that country; or who, not having a nationality and being outside the country of his former habitual residence, is unable or, owing to such fear, is unwilling to return to it.”
Compare 19 U.S.T. 6225 with 19 U.S.T. 6261. Not only did Congress adopt the Protocol’s standard in the statute, but there were also many statements indicating Congress’ intent that the new statutory definition of “refugee” be interpreted in conformance with the Protocol’s definition. The Conference Committee Report, for example, stated that the definition was accepted “with the understanding that it is based directly upon the language of the Protocol and it is intended that the provision be construed consistent with the Protocol.” S. Rep. No. 96-590, p. 20 (1980); see also H. R. Rep., at 9. It is thus appropriate to consider what the phrase “well-founded fear” means with relation to the Protocol.
The origin of the Protocol’s definition of “refugee” is found in the 1946 Constitution of the International Refugee Organization (IRO). See 62 Stat. 3037. The IRO defined a “refugee” as a person who had a “valid objection” to returning to his country of nationality, and specified that “fear, based on reasonable grounds of persecution because of race, religion, nationality, or political opinions ...” constituted a valid objection. See IRO Constitution, Annex 1, Pt. 1, § Cl(a)(i). The term was then incorporated in the United Nations Con[438]*438vention Relating to the Status of Refugees,20 189 U.N.T.S. 150 (July 28, 1951). The Committee that drafted the provision explained that “[t]he expression ‘well-founded fear of being the victim of persecution . . .’ means that a person has either been actually a victim of persecution or can show good reason why he fears persecution.” U. N. Rep., at 39. The 1967 Protocol incorporated the “well-founded fear” test, without modification. The standard, as it has been consistently understood by those who drafted it, as well as those drafting the documents that adopted it, certainly does not require an alien to show that it is more likely than not that he will be persecuted in order to be classified as a “refugee.”21
In interpreting the Protocol’s definition of “refugee” we are further guided by the analysis set forth in the Office of the [439]*439United Nations High Commissioner for Refugees, Handbook on Procedures and Criteria for Determining Refugee Status (Geneva, 1979).22 The Handbook explains that “[i]n general, the applicant’s fear should be considered well founded if he can establish, to a reasonable degree, that his continued stay-in his country of origin has become intolerable to him for the reasons stated in the definition, or would for the same reasons be intolerable if he returned there.” Id., at Ch. II B(2)(a) §42; see also id., §§37-41.
The High Commissioner’s analysis of the United Nations’ standard is consistent with our own examination of the origins of the Protocol’s definition,23 as well as the conclusions of [440]*440many scholars who have studied the matter.24 There is simply no room in the United Nations’ definition for concluding that because an applicant only has a 10% chance of being shot, tortured, or otherwise persecuted, that he or she has no “well-founded fear” of the event happening. See supra, at 431. As we pointed out in Stevic, a moderate interpretation of the “well-founded fear” standard would indicate “that so long as an objective situation is established by the evidence, it need not be shown that the situation will probably result in persecution, but it is enough that persecution is a reasonable possibility.” 467 U. S., at 424-425.
In Stevic, we dealt with the issue of withholding of deportation, or nonrefoulement, under § 243(h). This provision corresponds to Article 33.1 of the Convention.25 Significantly though, Article 33.1 does not extend this right to everyone who meets the definition of “refugee.” Rather, it provides that “[n]o Contracting State shall expel or return (‘refouler’) a refugee in any manner whatsoever to the frontiers or territories where his life or freedom would be threatened on account of his race, religion, nationality, membership or a particular social group or political opinion.” 19 U.S.T., at 6276,189 U.N.T.S., at 176 (emphasis added). Thus, Article 33.1 requires that an applicant satisfy two burdens: first, that he or she be a “refugee,” i. e., prove at least a “well-[441]*441founded fear of persecution”; second, that the “refugee” show that his or her life or freedom “would be threatened” if deported. Section 243(h)’s imposition of a “would be threatened” requirement is entirely consistent with the United States’ obligations under the Protocol.
Section 208(a), by contrast, is a discretionary mechanism which gives the Attorney General the authority to grant the broader relief of asylum to refugees. As such, it does not correspond to Article 33 of the Convention, but instead corresponds to Article 34. See Carvajal-Munoz, 743 F. 2d, at 574, n. 15. That Article provides that the contracting States “shall as far as possible facilitate the assimilation and naturalization of refugees. ...” Like § 208(a), the provision is prec-atory; it does not require the implementing authority actually to grant asylum to all those who are eligible. Also like § 208(a), an alien must only show that he or she is a “refugee” to establish eligibility for relief. No further showing that he or she “would be” persecuted is required.
Thus, as made binding on the United States through the Protocol, Article 34 provides for a precatory, or discretionary, benefit for the entire class of persons who qualify as “refugees,” whereas Article 33.1 provides an entitlement for the subcategory that “would be threatened” with persecution upon their return. This precise distinction between the broad class of refugees and the subcategory entitled to § 243(h) relief is plainly revealed in the 1980 Act. See Stevic, 467 U. S., at 428, n. 22.
Congress’ Rejection of S. 64.3.
Both the House bill, H. R. 2816, 96th Cong., 1st Sess. (1979), and the Senate bill, S. 643, 96th Cong., 1st Sess. (1979), provided that an alien must be a “refugee” within the meaning of the Act in order to be eligible for asylum. The two bills differed, however, in that the House bill authorized the Attorney General, in his discretion, to grant asylum to any refugee, whereas the Senate bill imposed the additional [442]*442requirement that a refugee could not obtain asylum unless “his deportation or return would be prohibited under section 243(h).”26 S. Rep., at 26. Although this restriction, if adopted, would have curtailed the Attorney General’s discretion to grant asylum to refugees pursuant to § 208(a), it would not have affected the standard used to determine whether an alien is a “refugee.” Thus, the inclusion of this prohibition in the Senate bill indicates that the Senate recognized that there is a difference between the “well-founded fear” standard and the clear-probability standard.27 The enactment of the House bill rather than the Senate bill in turn demonstrates that Congress eventually refused to restrict eligibility for asylum only to aliens meeting the stricter standard. “Few principles of statutory construction are more compelling than the proposition that Congress does not intend sub [443]*443silentio to enact statutory language that it has earlier discarded in favor of other language.” Nachman Corp. v. Pension Benefit Guaranty Corporation, 446 U. S. 359, 392-393 (1980) (Stewart, J., dissenting); cf. Gulf Oil Corp. v. Copp Paving Co., 419 U. S. 186, 200 (1974); Russello v. United States, 464 U. S., at 23.
IV
The INS makes two major arguments to support its contention that we should reverse the Court of Appeals and hold that an applicant can only show a “well-founded fear of persecution” by proving that it is more likely than not that he or she will be persecuted. We reject both of these arguments: the first ignores the structure of the Act; the second misconstrues the federal courts’ role in reviewing an agency’s statutory construction.
First, the INS repeatedly argues that the structure of the Act dictates a decision in its favor, since it is anomalous for § 208(a), which affords greater benefits than § 243(h), see n. 6, supra, to have a less stringent standard of eligibility. This argument sorely fails because it does not take into account the fact that an alien who satisfies the applicable standard under § 208(a) does not have a right to remain in the United States; he or she is simply eligible for asylum, if the Attorney General, in his discretion, chooses to grant it. An alien satisfying §243(h)’s stricter standard, in contrast, is automatically entitled to withholding of deportation.28 In Matter of Salim, 18 I. & N. Dec. 311 (1982), for example, the Board held that the alien was eligible for both asylum and withholding of deportation, but granted him the more limited remedy only, exercising its discretion to deny him asylum. See also Walai v. INS, 552 F. Supp. 998 (SDNY 1982); Mat[444]*444ter of Shirdel, Interim Decision No. 2958 (BIA Feb. 21, 1984). We do not consider it at all anomalous that out of the entire class of “refugees,” those who can show a clear probability of persecution are entitled to mandatory suspension of deportation and eligible for discretionary asylum, while those who can only show a well-founded fear of persecution are not entitled to anything, but are eligible for the discretionary relief of asylum.
There is no basis for the INS’s assertion that the discretionary/mandatory distinction has no practical significance. Decisions such as Matter of Salim, supra, and Matter of Shirdel, swpra, clearly demonstrate the practical import of the distinction. Moreover, the 1980 Act amended § 243(h) for the very purpose of changing it from a discretionary to a mandatory provision. See supra, at 428-429. Congress surely considered the discretionary/mandatory distinction important then, as it did with respect to the very definition of “refugee” involved here. The House Report provides:
“The Committee carefully considered arguments that the new definition might expand the numbers of refugees eligible to come to the United States and force substantially greater refugee admissions than the country could absorb. However, merely because an individual or group comes within the definition will not guarantee resettlement in the United States.” H. R. Rep., at 10.
This vesting of discretion in the Attorney General is quite typical in the immigration area, see, e. g., INS v. Jong Ha Wang, 450 U. S. 139 (1981). If anything is anomalous, it is that the Government now asks us to restrict its discretion to a narrow class of aliens. Congress has assigned to the Attorney General and his delegates the task of making these hard individualized decisions; although Congress could have crafted a narrower definition, it chose to authorize the At[445]*445torney General to determine which, if any, eligible refugees should be denied asylum.
The INS’s second principal argument in support of the proposition that the “well-founded fear” and “clear probability” standard are equivalent is that the BIA so construes the two standards. The INS argues that the BIA’s construction of the Refugee Act of 1980 is entitled to substantial deference, even if we conclude that the Court of Appeals’ reading of the statutes is more in keeping with Congress’ intent.29 This argument is unpersuasive.
[446]*446The question whether Congress intended the two standards to be identical is a pure question of statutory construction for the courts to decide. Employing traditional tools of statutory construction, we have concluded that Congress did not intend the two standards to be identical.30 In Chevron [447]*447U. S. A. Inc. v. Natural Resources Defense Council, Inc., 467 U. S. 837 (1984), we explained:
“The judiciary is the final authority-on issues of statutory construction and must reject administrative constructions which are contrary to clear congressional [448]*448intent. [Citing cases.] If a court, employing traditional tools of statutory construction, ascertains that Congress had an intention on the precise question at issue, that intention is the law and must be given effect.” Id., at 843, n. 9 (citations omitted).
The narrow legal question whether the two standards are the same is, of course, quite different from the question of interpretation that arises in each case in which the agency is required to apply either or both standards to a particular set of facts. There is obviously some ambiguity in a term like “well-founded fear” which can only be given concrete meaning through a process of case-by-case adjudication. In that process of filling “‘any gap left, implicitly or explicitly, by Congress,’” the courts must respect the interpretation of the agency to which Congress has delegated the responsibility for administering the statutory program. See Chevron, supra, at 843, quoting Morton v. Ruiz, 415 U. S. 199, 231 (1974). But our task today is much narrower, and is well within the province of the Judiciary. We do not attempt to set forth a detailed description of how the “well-founded fear” test should be applied.31 Instead, we merely hold that the Immigration Judge and the BIA were incorrect in holding that the two standards are identical.32
[449]*449Our analysis of the plain language of the Act, its symmetry with the United Nations Protocol, and its legislative history, lead inexorably to the conclusion that to show a “well-founded fear of persecution,” an alien need not prove that it is more likely than not that he or she will be persecuted in his or her home country. We find these ordinary canons of statutory construction compelling, even without regard to the longstanding principle of construing any lingering ambiguities in deportation statutes in favor of the alien. See INS v. Errico, 385 U. S. 214, 225 (1966); Costello v. INS, 376 U. S. 120, 128 (1964); Fong Haw Tan v. Phelan, 333 U. S. 6, 10 (1948).
Deportation is always a harsh measure; it is all the more replete with danger when the alien makes a claim that he or she will be subject to death or persecution if forced to return to his or her home country. In enacting the Refugee Act of 1980 Congress sought to “give the United States sufficient flexibility to respond to situations involving political or religious dissidents and detainees throughout the world.” H. R. Rep., at 9. Our holding today increases that flexibility by rejecting the Government’s contention that the Attorney General may not even consider granting asylum to one who [450]*450fails to satisfy the strict § 243(h) standard. Whether or not a “refugee” is eventually granted asylum is a matter which Congress has left for the Attorney General to decide. But it is clear that Congress did not intend to restrict eligibility for that relief to those who could prove that it is more likely than not that they will be persecuted if deported.
The judgment of the Court of Appeals is
Affirmed.