opinion: Justice Kennedy
  delivered the opinion of the Court.
  The State of Michigan exempts from taxation all retirement benefits paid by the State or its political subdivisions, but levies an income tax on retirement benefits paid by all other employers, including the Federal Government. The question presented by this case is whether Michigan’s tax scheme violates federal law.
  I
  Appellant Paul S. Davis, a Michigan resident, is a former employee of the United States Government. He receives retirement benefits pursuant to the Civil Service Retirement Act, 5 U. S. C. §8831 et seq. In each of the years 1979 through 1984, appellant paid Michigan state income tax on his federal retirement benefits in accordance with Mich. Comp. Laws Ann. §206.30(l)(f) (Supp. 1988). That statute defines taxable income in a manner that excludes all retirement benefits received from the State or its political subdivisions, but includes most other forms of retirement benefits. The effect of this definition is that the retirement benefits of retired state employees are exempt from state taxation while the benefits received by retired federal employees are not.
  In 1984, appellant petitioned for refunds of state taxes paid on his federal retirement benefits between 1979 and 1983. After his request was denied, appellant filed suit in the Michigan Court of Claims. Appellant’s complaint, which was amended to include the 1984 tax year, averred that his federal retirement benefits were “not legally taxable under the Michigan Income Tax Law” and that the State’s inconsistent treatment of state and federal retirement benefits discriminated against federal retirees in violation of 4 U. S. C. § 111, which preserves federal employees’ immunity from discriminatory state taxation. See Public Salary Tax Act of 1939, ch. 59, §4, 53 Stat. 575, codified, as amended, at 4 U. S. C. § 111. The Court of Claims, however, denied relief. No. 84-9451 (Oct. 30, 1985), App. to Juris. Statement A10.
  The Michigan Court of Appeals affirmed. 160 Mich. App. 98, 408 N. W. 2d 433 (1987). The court first rejected appellant’s claim that 4 U. S. C. § 111 invalidated the State’s tax on appellant’s federal benefits. Noting that §111 applies only to federal “employees,” the court determined that appellant’s status under federal law was that of an “annuitant” rather than an employee. As a consequence, the court concluded that § 111 “has no application to [Davis], since [he] cannot be considered an employee within the meaning of that act.” Id., at 104, 408 N. W. 2d, at 435.
  The Michigan Court of Appeals next rejected appellant’s contention that the doctrine of intergovernmental tax immunity rendered the State’s tax treatment of federal retirement benefits unconstitutional. Conceding that “a tax may be held invalid ... if it operates to discriminate against the federal government and those with whom it deals,” id., at 104, 408 N. W. 2d, at 436, the court examined the State’s justifications for the discrimination under a rational-basis test. Ibid. The court determined that the State’s interest in “attracting and retaining . . . qualified employees” was a “legitimate state objective which is rationally achieved by a retirement plan offering economic inducements,” and it upheld the statute. Id., at 105, 408 N. W. 2d, at 436.
  The Supreme Court of Michigan denied appellant’s application for leave to appeal. 429 Mich. 854 (1987). We noted probable jurisdiction. 487 U. S. 1217 (1988).
  II
  Appellant places principal reliance on 4 U. S. C. § 111. In relevant part, that section provides:
  “The United States consents to the taxation' of pay or compensation for personal service as an officer or employee of the United States ... by a duly constituted taxing authority having jurisdiction, if the taxation does not discriminate against the officer or employee because of the source of the pay or compensation.”
  As a threshold matter, the State argues that § 111 applies only to current employees of the Federal Government, not to retirees such as appellant. In our view, however, the plain language of the statute dictates the opposite conclusion. Section 111 by its terms applies to “the taxation of pay or compensation for personal services as an officer or employee of the United States.” (Emphasis added). While retirement pay is not actually disbursed during the time an individual is working for the Government, the amount of benefits to be received in retirement is based and computed upon the individual’s salary and years of service. 5 U. S. C. § 8339(a). We have no difficulty concluding that civil service retirement benefits are deferred compensation for past years of service rendered to the Government. See, e. g., Zucker v. United States, 758 F. 2d 637, 639 (CA Fed.), cert. denied, 474 U. S. 842 (1985); Kizas v. Webster, 227 U. S. App. D. C. 327, 339, 707 F. 2d 524, 536, (1983), cert. denied, 464 U. S. 1042 (1984); Clark v. United States, 691 F. 2d 837, 842 (CA7 1982). And because these benefits accrue to employees on account of their service to the Government, they fall squarely within the category of compensation for services rendered “as an officer or employee of the United States.” Appellant’s federal retirement benefits are deferred compensation earned “as” a federal employee, and so are subject to § 111.
  The State points out, however, that the reference to “compensation for personal services as an officer or employee” occurs in the first part of § 111, which defines the extent of Congress’ consent to state taxation, and not in the latter part of the section, which provides that the consent does not extend to taxes that discriminate against federal employees. Instead, the nondiscrimination clause speaks only in terms of “discriminat[ion] against the officer or employee because of the source of the pay or compensation.” From this the State concludes that, whatever the scope of Congress’ consent to taxation in the first portion of § 111, the nondiscrimination clause applies only to current federal employees.
  Although the State’s hypertechnical reading of the nondiscrimination clause is not inconsistent with the language of that provision examined in isolation, statutory language cannot be construed in a vacuum. It is a fundamental canon of statutory construction that the words of a statute must be read in their context and with a view to their place in the overall statutory scheme. See United States v. Morton, 467 U. S. 822, 828 (1984). When the first part of §111 is read together with the nondiscrimination clause, the operative words of the statute are as follows: “The United States consents to the taxation of pay or compensation ... if the taxation does not discriminate . . . because of the source of the pay or compensation.” The reference to “the pay or compensation” in the last clause of § 111 must, in context, mean the same “pay or compensation” defined in the first part of the section. Since that “pay or compensation” includes retirement benefits, the nondiscrimination clause must include them as well.
  Any other interpretation of the nondiscrimination clause would be implausible at best. It is difficult to imagine that Congress consented to discriminatory taxation of the pensions of retired federal civil servants while refusing to permit such taxation of current employees, and nothing in the statutory language or even in the legislative history suggests this result. While Congress could perhaps have used more precise language, the overall meaning of § 111 is unmistakable: it waives whatever immunity past and present federal employees would otherwise enjoy from state taxation of salaries, retirement benefits, and other forms of compensation paid on account of their employment with the Federal Government, except to the extent that such taxation discriminates on account of the source of the compensation.
  1 — 1 HH hH
  Section 111 was enacted as part of the Public Salary Tax Act of 1939, the primary purpose of which was to impose federal income tax on the salaries of all state and local government employees. Prior to adoption of the Act, salaries of most government employees, both state and federal, generally were thought to be exempt from taxation by another sovereign under the doctrine of intergovernmental tax immunity. This doctrine had its genesis in McCulloch v. Maryland, 4 Wheat. 316 (1819), which held that the State of Maryland could not impose a discriminatory tax on the Bank of the United States. Chief Justice Marshall’s opinion for the Court reasoned that the Bank was an instrumentality of the Federal Government used to carry into effect the Government’s delegated powers, and taxation by the State would unconstitutionally interfere with the exercise of those powers. Id., at 425-437.
  For a time, McCulloch was read broadly to bar most taxation by one sovereign of the employees of another. See Collector v. Day, 11 Wall. 113, 124-128 (1871) (invalidating federal income tax on salary of state judge); Dobbins v. Com missioners of Erie County, 16 Pet. 435 (1842) (invalidating state tax on federal officer). This rule “was based on the rationale that any tax on income a party received under a contract with the government was a tax on the contract and thus a tax ‘on’ the government because it burdened the government’s power to enter into the contract.” South Carolina v. Baker, 485 U. S. 505, 518 (1988).
  In subsequent cases, however, the Court began to turn away from its more expansive applications of the immunity doctrine. Thus, in Helvering v. Gerhardt, 304 U. S. 405 (1938), the Court held that the Federal Government could levy nondiscriminatory taxes on the incomes of most state employees. The following year, Graves v. New York ex rel. O’Keefe, 306 U. S. 466, 486-487 (1939), overruled the Day-Dobbins line of cases that had exempted government employees from nondiscriminatory taxation. After Graves, therefore, intergovernmental tax immunity barred only those taxes that were imposed directly on one sovereign by the other or that discriminated against a sovereign or those with whom it dealt.
  It was in the midst of this judicial revision of the immunity doctrine that Congress decided to extend the federal income tax to state and local government employees. The Public Salary Tax Act was enacted after Helvering v. Gerhardt, supra, had upheld the imposition of federal income taxes on state civil servants, and Congress relied on that decision as support for its broad assertion of federal taxing authority. S. Rep. No. 112, 76th Cong., 1st Sess., 5-9 (1939); H. R. Rep. No. 26, 76th Cong., 1st Sess., 2-3 (1939). However, the Act was drafted, considered in Committee, and passed by the House of Representatives before the announcement of the decision in Graves v. New York ex rel. O’Keefe, supra, which for the first time permitted state taxation of federal employees. As a result, during most of the legislative process leading to adoption of the Act it was unclear whether state taxation of federal employees was still barred by intergovernmental tax immunity despite the abrogation of state employees’ immunity from federal taxation. See H. R. Rep. No. 26, swpra, at 2 (“There are certain indications in the case of McCulloch v. Maryland, 4 Wheat. 316 (1819), . . . that . . . Federal officers and employees may not, without the consent of the United States, be subjected to income taxation under the authority of the various States”).
  Dissatisfied with this uncertain state of affairs, and concerned that considerations of fairness demanded equal tax treatment for state and federal employees, Congress decided to ensure that federal employees would not remain immune from state taxation at the same time that state government employees were being required to pay federal income taxes. See S. Rep. No. 112, supra, at 4; H. R. Rep. No. 26, supra, at 2. Accordingly, §4 of the proposed Act (now §111) expressly waived whatever immunity would have otherwise shielded federal employees from nondiscriminatory state taxes.
  By the time the statute was enacted, of course, the decision in Graves had been announced, so the constitutional immunity doctrine no longer proscribed nondiscriminatory state taxation of federal employees. In effect, § 111 simply codified the result in Graves and foreclosed the possibility that subsequent judicial reconsideration of that case might reestablish the broader interpretation of the immunity doctrine.
  Section 111 did not waive all aspects of intergovernmental tax immunity, however. The final clause of the section contains an exception for state taxes that discriminate against federal employees on the basis of the source of their compensation. This nondiscrimination clause closely parallels the nondiscrimination component of the constitutional immunity doctrine which has, from the time of McCulloch v. Maryland, barred taxes that “operat[e] so as to discriminate against the Government or those with whom it deals.” United States v. City of Detroit, 355 U. S. 466, 473 (1958). See also McCulloch v. Maryland, supra, at 436-437; Miller v. Milwaukee, 272 U. S. 713, 714-715 (1927); Helvering v. Gerhardt, supra, at 413; Phillips Chemical Co. v. Dumas Independent School Dist., 361 U. S. 376, 385 (1960); Memphis Bank & Trust Co. v. Gamer, 459 U. S. 392, 397, and n. 7 (1983).
  In view of the similarity of language and purpose between the constitutional principle of nondiscrimination and the statutory nondiscrimination clause, and given that § 111 was consciously drafted against the background of the Court’s tax immunity cases, it is reasonable to conclude that Congress drew upon the constitutional doctrine in defining the scope of the immunity retained in § 111. When Congress codifies a judicially defined concept, it is presumed, absent an express statement to the contrary, that Congress intended to adopt the interpretation placed on that concept by the courts. See Midlantic National Bank v. New Jersey Dept. of Environmental Protection, 474 U. S. 494, 501 (1986); Morissette v. United States, 342 U. S. 246, 263 (1952). Hence, we conclude that the retention of immunity in § 111 is coextensive with the prohibition against discriminatory taxes embodied in the modern constitutional doctrine of intergovernmental tax immunity. Cf. Memphis Bank & Trust, supra, at 396-397 (construing 31 U. S. C. §742, which permits only ‘“nondiscriminatory’” state taxation of interest on federal obligations, as “principally a restatement of the constitutional rule”).
  On its face, § 111 purports to be nothing more than a partial congressional consent to nondiscriminatory state taxation of federal employees. It can be argued, however, that by negative implication §111 also constitutes an affirmative statutory grant of immunity from discriminatory state taxation in addition to, and coextensive with, the pre-existing protection afforded by the constitutional doctrine. Regardless of whether § 111 provides an independent basis for finding immunity or merely preserves the traditional constitutional prohibition against discriminatory taxes, however, the inquiry is the same. In either case, the scope of the immunity granted or retained by the nondiscrimination clause is to be determined by reference to the constitutional doctrine. Thus, the dispositive question in this case is whether the tax imposed on appellant is barred by the doctrine of intergovernmental tax immunity.
  IV
  It is undisputed that Michigan’s tax system discriminates in favor of retired state employees and against retired federal employees. The State argues, however, that appellant is not entitled to claim the protection of the immunity doctrine, and that in any event the State’s inconsistent treatment of Federal and State Government retirees is justified by meaningful differences between the two classes.
  A
  In support of its first contention, the State points out that the purpose of the immunity doctrine is to protect governments and not private entities or individuals. As a result, so long as the challenged tax does not interfere with the Federal Government’s ability to perform its governmental functions, the constitutional doctrine has not been violated.
  It is true that intergovernmental tax immunity is based on the need to protect each sovereign’s governmental operations from undue interference by the other. Graves, 306 U. S., at 481; McCulloch v. Maryland, 4 Wheat., at 435-436. But it does not follow that private entities or individuals who are subjected to discriminatory taxation on account of their dealings with a sovereign cannot themselves receive the protection of the constitutional doctrine. Indeed, all precedent is to the contrary. In Phillips Chemical Co., supra, for example, we considered a private corporation’s claim that a state tax discriminated against private lessees of federal land. We concluded that the tax “discriminate[d] unconstitutionally against the United States and its lessee,” and accordingly held that the tax could not be exacted. Id., at 387 (emphasis added). See also Memphis Bank & Trust, supra; Moses Lake Homes, Inc. v. Grant County, 365 U. S. 744 (1961); Collector v. Day, 11 Wall. 113 (1871); Dobbins v. Commissioners of Erie County, 16 Pet. 435 (1842). The State offers no reasons for departing from this settled rule, and we decline to do so.
  B
  Under our precedents, “[t]he imposition of a heavier tax burden on [those who deal with one sovereign] than is imposed on [those who deal with the other] must be justified by significant differences between the two classes.” Phillips Chemical Co. v. Dumas Independent School Dist., 361 U. S., at 383. In determining whether this standard of justification has been met, it is inappropriate to rely solely on the mode of analysis developed in our equal protection cases. We have previously observed that “our decisions in [the equal protection] field are not necessarily controlling where problems of intergovernmental tax immunity are involved,” because “the Government’s interests must be weighed in the balance.” Id., at 385. Instead, the relevant inquiry is whether the inconsistent tax treatment is directly related to, and justified by, “significant differences between the two classes.” Id., at 383-385.
  The State points to two allegedly significant differences between federal and state retirees. First, the State suggests that its interest in hiring and retaining qualified civil servants through the inducement of a tax exemption for retirement benefits is sufficient to justify the preferential treatment of its retired employees. This argument is wholly beside the point, however, for it does nothing to demonstrate that there are “significant differences between the two classes” themselves; rather, it merely demonstrates that the State has a rational reason for discriminating between two similar groups of retirees. The State’s interest in adopting the discriminatory tax, no matter how substantial, is simply irrelevant to an inquiry into the nature of the two classes receiving inconsistent treatment. See id., at 384.
  Second, the State argues that its retirement benefits are significantly less munificent than those offered by the Federal Government, in terms of vesting requirements, rate of accrual, and computation of benefit amounts. The substantial differences in the value of the retirement benefits paid the two classes should, in the State’s view, justify the inconsistent tax treatment.
  Even assuming the State’s estimate of the relative value of state and federal retirement benefits is generally correct, we do not believe this difference suffices to justify the type of blanket exemption at issue in this case. While the average retired federal civil servant receives a larger pension than his state counterpart, there are undoubtedly many individual instances in which the opposite holds true. A tax exemption truly intended to account for differences in retirement benefits would not discriminate on the basis of the source of those benefits, as Michigan’s statute does; rather, it would discriminate on the basis of the amount of benefits received by individual retirees. Cf. Phillips Chemical Co., supra, at 384-385 (rejecting proffered rationale for State’s unfavorable tax treatment of lessees of federal property, because an evenhanded application of the rationale would have resulted in inclusion of some lessees of state property in the disfavored class as well).
  V
  For these reasons, we conclude that the Michigan Income Tax Act violates principles of intergovernmental tax immunity by favoring retired state and local government employees over retired federal employees. The State having conceded that a refund is appropriate in these circumstances, see Brief for Appellee 63, to the extent appellant has paid taxes pursuant to this invalid tax scheme, he is entitled to a refund. See Iowa-Des Moines National Bank v. Bennett, 284 U. S. 239, 247 (1931).
  Appellant also seeks prospective relief from discriminatory taxation. With respect to this claim, however, we are not in the best position to ascertain the appropriate remedy. While invalidation of Michigan’s income tax law in its entirety obviously would eliminate the constitutional violation, the Constitution does not require such a drastic solution. We have recognized, in cases involving invalid classifications in the distribution of government benefits, that the appropriate remedy “is a mandate of equal treatment, a result that can be accomplished by withdrawal of benefits from the favored class as well as by extension of benefits to the excluded class.” Heckler v. Mathews, 465 U. S. 728, 740 (1984). See Iowa-Des Moines National Bank, supra, at 247; see also Welsh v. United States, 398 U. S. 333, 361 (1970) (Harlan, J., concurring in judgment).
  In this case, appellant’s claim could be resolved either by extending the tax exemption to retired federal employees (or to all retired employees), or by eliminating the exemption for retired state and local government employees. The latter approach, of course, could be construed as the direct imposition of a state tax, a remedy beyond the power of a federal court. See Moses Lake Homes, Inc. v. Grant County, 365 U. S., at 752 (“Federal courts may not assess or levy taxes”). The permissibility of either approach, moreover, depends in part on the severability of a portion of §206.30(l)(f) from the remainder of the Michigan Income Tax Act, a question of state law within the special expertise of the Michigan courts. See Louis K. Liggett Co. v. Lee, 288 U. S. 517, 540-541 (1933). It follows that the Michigan courts are in the best position to determine how to comply with the mandate of equal treatment. The judgment of the Court of Appeals is reversed, and the case is remanded for further proceedings not inconsistent with this opinion.
  It is so ordered.
  As a result of a series of amendments, this subsection has been variously designated as (l)(f), (l)(g), and (l)(h) at times relevant to this litigation. This opinion will refer only to the current statutory designation, §206.30(l)(f).
  In pertinent part, the statute provides:
  “(1) ‘Taxable income’. . . means adjusted gross income as defined in the internal revenue code subject to the following adjustments:
  “(f) Deduct to the extent included in adjusted gross income:
  “(i) Retirement or pension benefits received from a public retirement system of or created by an act of this state or a political subdivision of this state.
  “(iv) Retirement or pension benefits from any other retirement or pension system as follows:
  “(A) For a single return, the sum of not more than $7,500.00.
  “(B) For a joint return, the sum of not more than $10,000.00.” Mich. Comp. Laws Ann. § 206.30(l)(f) (Supp. 1988).
  Subsection (f)(iv) of this provision exempts a portion of otherwise taxable retirement benefits from taxable income, but appellant’s retirement pay from all nonstate sources exceeded the applicable exemption amount in each of the tax years relevant to this case.
  The State suggests that the legislative history does not support this interpretation of § 111, pointing to statements in the Committee Reports that describe the scope of § 111 without using the phrase “service as an officer or employee.” The language of the statute leaves no room for doubt on this point, however, so the State’s attempt to establish a minor inconsistency with the legislative history need not detain us. Legislative history is irrelevant to the interpretation of an unambiguous statute. United Air Lines, Inc. v. McMann, 434 U. S. 192, 199 (1977).
  The dissent argues that this tax is nondiscriminatory, and thus constitutional, because it “draws no distinction between the federal employees or retirees and the vast majority of voters in the State.” Post, at 823. In Phillips Chemical Co., however, we faced that precise situation: an equal tax burden was imposed on lessees of private, tax-exempt property and lessees of federal property, while lessees of state property paid a lesser tax, or in some circumstances none at all. Although we concluded that “[ujnder these circumstances, there appears to be no discrimination between the Government’s lessees and lessees of private property,” 361 U. S., at 381, we nonetheless invalidated the State’s tax. This result is consistent with the underlying rationale for the doctrine of intergovernmental tax immunity. The danger that a State is engaging in impermissible discrimination against the Federal Government is greatest when the State acts to benefit itself and those in privity with it. As we observed in Phillips Chemical Co., “it does not seem too much to require that the State treat those who deal with the Government as well as it treats those with whom it deals itself.” Id.., at 385.
  We also take issue with the dissent’s assertion that “it is peculiarly inappropriate to focus solely on the treatment of state governmental employees” because “[t]he State may always compensate in pay or salary for what it assesses in taxes.” Post, at 824. In order to provide the same after-tax benefits to all retired state employees by means of increased salaries or benefit payments instead of a tax exemption, the State would have to increase its outlays by more than the cost of the current tax exemption, since the increased payments to retirees would result in higher federal income tax payments in some circumstances. This fact serves to illustrate the impact on the Federal Government of the State’s discriminatory tax exemption for state retirees. Taxes enacted to reduce the State’s employment costs at the expense of the federal treasury are the type of discriminatory legislation that the doctrine of intergovernmental tax immunity is intended to bar.
choices: Alabama Alaska American Samoa Arizona Arkansas California Colorado Connecticut Delaware District of Columbia Federated States of Micronesia Florida Georgia Guam Hawaii Idaho Illinois Indiana Iowa Kansas Kentucky Louisiana Maine Marshall Islands Maryland Massachusetts Michigan Minnesota Mississippi Missouri Montana Nebraska Nevada New Hampshire New Jersey New Mexico New York North Carolina North Dakota Northern Mariana Islands Ohio Oklahoma Oregon Palau Pennsylvania Puerto Rico Rhode Island South Carolina South Dakota Tennessee Texas Utah Vermont Virgin Islands Virginia Washington West Virginia Wisconsin Wyoming United States Interstate Compact Philippines Indian Dakota