The Appointments Clause of the Constitution generally requires high-level "officers of the United States" to be appointed through nomination by the President, with the advice and consent of the Senate. However, appointment to these advice and consent positions can be a lengthy process, and officers sometimes unexpectedly vacate offices, whether by resignation, death, or other absence, leaving before a successor has been chosen. In particular, there are often a large number of vacancies during a presidential transition, when a new President seeks to install new officers in important executive positions. The most recent transition of Administrations was no exception, and reports have noted that a number of offices across the executive branch currently remain vacant. In the case of such a vacancy, Congress has long provided that individuals who were not appointed to that office may temporarily perform the functions of that office. Generally, to serve as an acting officer for an advice and consent position, a government officer or employee must be authorized to perform the duties of a vacant office by the Federal Vacancies Reform Act of 1998 (Vacancies Act). The Vacancies Act allows only certain classes of employees to serve as an acting officer for an advice and consent position, and specifies that they may serve for only a limited period. If a covered acting officer's service is not authorized by the Vacancies Act, any attempt by that officer to perform a "function or duty" of a vacant office has "no force or effect." This report first describes how the Vacancies Act operates and outlines its scope, identifying when the Vacancies Act applies to a given office, how it is enforced, and which offices are exempt from its provisions. The report then explains who may serve as an acting officer and for how long, focusing on the limitations the Vacancies Act places on acting service. Finally, the report turns to issues of particular relevance to Congress, primarily highlighting the Vacancies Act's enforcement mechanisms. The Vacancies Act generally provides "the exclusive means for temporarily authorizing an acting official to perform the functions and duties of any office of an Executive agency . . . for which appointment is required to be made by the President, by and with the advice and consent of the Senate." The Vacancies Act's requirements are triggered if an officer serving in an advice and consent position in the executive branch "dies, resigns, or is otherwise unable to perform the functions and duties of the office." Because the Vacancies Act is generally exclusive and subject to limited exceptions, a person may not temporarily perform "the functions and duties" of a vacant advice and consent position unless that service comports with the Vacancies Act. The Vacancies Act specifies that a "function or duty" is one that, by statute or regulation, must be performed by the officer in question. Section 3348 provides that, "unless an officer or employee is performing the functions and duties [of an office] in accordance with" the Act, "the office shall remain vacant." If there is no acting officer serving in compliance with the Vacancies Act, then generally "only the head of [an agency] may perform" the functions and duties of that vacant office. As a result, Section 3348 usually allows three types of people to perform the functions and duties of an advice and consent office when it is vacant: the agency head, a person complying with the Vacancies Act, or a person complying with another statute that allows acting service. Section 3348 further provides that "an action taken by any person who" is not complying with the Vacancies Act "in the performance of any function or duty of a vacant office . . . shall have no force or effect." The Supreme Court has suggested that the Vacancies Act renders any noncompliant actions "void ab initio ," meaning that the action was "null from the beginning." The consequences that flow from a determination that an action is "void" are more severe than if a court were to announce that the action was merely "voidable." A "voidable" action is one that may be judged invalid because of some legal defect, but that "is not incurable." For instance, before a court strikes down a voidable agency decision, it will often inquire into whether the legal defect created actual prejudice. If an error is harmless, the court may uphold the agency action. Critically, acts that are "void" may not be ratified or rendered harmless, meaning that another person who properly exercises legal authority on behalf of an agency may not subsequently approve or replicate the act, thereby rendering it valid. The Vacancies Act affirms this consequence by explicitly specifying that an agency may not ratify any acts taken in violation of the statute. As is discussed in more detail later in this report, the Vacancies Act has primarily been enforced through the courts, when a person with standing challenges an agency action on the basis that it was undertaken by an officer who was performing a function or duty of a vacant office in violation of the Vacancies Act. If such a challenge is successful, a court would be likely to vacate the challenged agency action. The Vacancies Act generally applies to advice and consent positions in executive agencies. The term "Executive agency" is defined broadly in Title 5 of the U.S. Code to mean "an Executive department, a Government corporation, [or] an independent establishment." However, the Vacancies Act explicitly excludes certain offices altogether. First, the Vacancies Act does not apply to officers of "the Government Accountability Office." Second, a distinct provision states that the Vacancies Act does not apply to (1) a member of a multimember board that "governs an independent establishment or Government corporation"; (2) a "commissioner of the Federal Energy Regulatory Commission"; (3) a "member of the Surface Transportation Board"; or (4) a federal judge serving in "a court constituted under article I of the United States Constitution." Additionally, while not excluded from the other requirements of the Vacancies Act, certain offices are exempt from the provision allowing only agency heads to perform the duties of a vacant office and the provision that renders noncompliant actions void. Specifically, Section 3348(e) states that "this section"—Section 3348—"shall not apply to" (1) the General Counsel of the National Labor Relations Board; (2) the General Counsel of the Federal Labor Relations Authority; (3) any Inspector General appointed by the President, by and with the advice and consent of the Senate; (4) any Chief Financial Officer appointed by the President, by and with the advice and consent of the Senate; or (5) an office of an Executive agency (including the Executive Office of the President, and other than the Government Accountability Office) if a statutory provision expressly prohibits the head of the Executive agency from performing the functions and duties of such office. The legislative history of the Vacancies Act sheds some light on the purpose of this exemption, suggesting that Congress sought to exclude these "unusual positions" from Section 3348 because these officials are meant to be "independent" of the commission or agency in which they serve. The Senate report accompanying the Act suggests that Congress intended "to separate the official who would investigate and charge potential violations of the underlying regulatory statute from the officials who would determine whether that statute had actually been violated." Allowing the head of the agency—or the commissioners—to perform the nondelegable duties of these positions would undermine the independence of these positions. It is not entirely clear what the consequences are if an acting officer in one of these exempt positions violates the Vacancies Act. Because Section 3348 does not apply to those positions, it appears that any noncompliant actions should not be rendered void. Instead, a court might conclude that any noncompliant acts are merely voidable—or could conclude that even if these officers violate the Vacancies Act, that law will not invalidate their actions. In NLRB v. SW General, Inc. , the Supreme Court held that the service of the Acting General Counsel of the National Labor Relations Board (NLRB) violated the Vacancies Act, but noted that this position was exempt "from the general rule that actions taken in violation of the [Vacancies Act] are void ab initio ." The Court affirmed the D.C. Circuit's ruling vacating the Acting General Counsel's noncompliant actions, but did not explicitly reconsider the issue of remedy. The D.C. Circuit in S.W. General, Inc. had itself clarified that it was not fully exploring the question of the appropriate remedy and was merely assuming , on the basis of the parties' arguments, "that section 3348(e)(1) renders the actions of an improperly serving Acting General Counsel voidable , not void." Because the D.C. Circuit assumed that the contested actions were voidable rather than void, the court considered but ultimately rejected two legal doctrines—the harmless error and de facto officer doctrine—that could have allowed the court to uphold the NLRB's action. If the Acting General Counsel were not exempt from Section 3348 and his noncompliance with the Vacancies Act had rendered his acts void ab initio , the court could not have considered whether any other legal doctrines cured the initial legal error with the Acting General Counsel's actions. Finally, the Vacancies Act contemplates that other statutes may, under limited circumstances, either supplement or supersede its provisions. Section 3347 provides that the Vacancies Act is exclusive unless "a statutory provision expressly" authorizes "an officer or employee to perform the functions and duties of a specified office temporarily in an acting capacity." However, Section 3347 states that a general statute authorizing the head of an executive agency "to delegate duties statutorily vested in that agency head to, or to reassign duties among, officers or employees of such Executive agency" will not supersede the limitations of the Vacancies Act on acting service. For instance, 28 U.S.C.