This article studies the concept of “mass incarceration,” the most relevant feature of criminal justice policy during the late twentieth and early twenty-first centuries. It first acknowledges the argument that the temporal and spatial borders of mass incarceration are still not understood properly, and then examines the expansion of U.S. prisons and jails that occurred from 1972 to 2010. From here the discussion turns to the institutions that are responsible for mass incarceration, such as prosecutors and legislations. The next section takes a look at the efforts to make sense of the arrival of mass incarceration as policy and/or politics. This article emphasizes that while the period of mass incarceration as a public policy has ended, its history as a social problem has just begun.