A U.S. state is any one of the fifty subnational entities of the United States, although four states use the official title "commonwealth". The separate state governments and the federal government share sovereignty, in that an American is a citizen both of the federal entity and of their state of domicile. However, state citizenship is very flexible, and no government approval is required to move between states (with the exception of convicts on parole).

The United States Constitution allocates power between the two levels of government in general terms. By ratifying the Constitution, each state transfers certain sovereign powers to the federal government. Under the Tenth Amendment, all powers not explicitly transferred are retained by the states and the people. Historically, the tasks of public education, public health, transportation and other infrastructure have been considered primarily state responsibilities, although all have significant federal funding and regulation as well.

Over time, the Constitution has been amended, and the interpretation and application of its provisions have changed. The general tendency has been toward centralization, with the federal government playing a much larger role than it once did. There is a continuing debate over "states' rights", which concerns the extent and nature of the states' powers and sovereignty in relation to that of the federal government.