Municipal annexation is a process by which a municipality expands its boundaries into nearby, usually adjacent, unincorporated areas. This has been a common response of cities to urbanization in neighboring areas. It may be done because the neighboring urban areas seek municipal services or because a city seeks control over its suburbs or neighboring unincorporated areas.

In the United States, all local governments are considered "creatures of the state" according to Dillon's Rule, which resulted from the work of John Forrest Dillon on the law of municipal corporations. Dillon's Rule implies, among other things, that the boundaries of any jurisdiction falling under state government can be modified by state government action. For this reason, examples of municipal annexation are distinct from annexations involving sovereign states.