In public policy, a sunset provision or sunset clause is a measure within a statute, regulation, or other law that provides for the law to cease to be effective after a specified date, unless further legislative action is taken to extend it. Unlike most laws that remain in force indefinitely unless they are amended or repealed, sunset provisions have a specified expiration date. Desuetude renders a law invalid after long non-use.

The roots of sunset provisions are laid in Roman law of the mandate, but the first philosophical reference is traced in the laws of Plato. At the time of the Roman Republic, the empowerment of the Roman Senate to collect special taxes and to activate troops was limited in time and extent. Those empowerments ended before the expiration of an electoral office, such as the Proconsul. The rule Ad tempus concessa post tempus censetur denegata is translated as "what is admitted for a period will be refused after the period". The same rules were applied in the Roman emergency legislation. The fundamental principle appeared in several areas of legislation and was later codified in the Codex Iustinianus (10, 61, 1). The principle was broken when Julius Caesar became dictator for life.

Sunset provisions have been used extensively throughout legal history. The idea of general sunset provisions was discussed extensively in the late 1970s. Sunset clauses with an effective extension review process have been argued as a safeguard of democracy to ensure emergency provisions, such as state of emergency, remain temporary. An increase in electoral accountability can be achieved with brief reviews resulting in a majority of provisions extended with no or cosmetic modifications and a record who advocates for extending the provisions. Sunset clauses with automatic expiration can reduce legal certainty and circumvent long-term budget constraints and regulatory impact analysis. Experimental regulations can test temporarily new legislative approaches.

Sunset provisions were a frequent legislative tool used by the colonial and early state legislatures but would decrease in popularity as the legislatures were institutionalized.

In American federal law parlance, legislation that renews an expired mandate is a reauthorization act or extension act. Extensive political wrangling often precedes reauthorizations of controversial laws or agencies. High-profile examples in American law include:

Article I, Section 8, which enumerates the powers of Congress, includes a sunset provision for expenditures on “Armies,” but not the Navy:

The Congress shall have Power
[…]
To raise and support Armies, but no Appropriation of Money to that Use shall be for a longer Term than two Years;
To provide and maintain a Navy;
[…]

Article V contains a provision “that no Amendment which may be made prior to the Year One thousand eight hundred and eight shall in any Manner affect the first and fourth Clauses in the Ninth Section of the first Article,” which, by its words, had sunsetted by 1808.