The size of the U.S. federal government, as well as state and local governments, increased dramatically during the 20th century. This paper reviews several theories of government size and growth that are dominant in the public choice and political science literature. The theories are divided into two categories: citizen-over-state theories and state-over-citizen theories. The relationship between the 16th Amendment to the U.S. Constitution and the timing of government growth is also presented. It is likely that portions of each theory can explain government size and growth, but the challenge facing economists is to develop a single unifying theory of government growth.