This manuscript provides an overview of state-level research on penal change in the United States in the latter half of the twentieth century. It outlines how research focused on developments in the states provides unique insights into the forces that helped drive mass incarceration and has generated rich historical accounts of how mass incarceration emerged within specific contexts. It charts some of the new theorizing that has emerged from state case studies, including those emphasizing how penal change is characterized by conflict and others that conceptualize state developments as they interact with federal processes. These insights are then considered against broader national theories of punishment. The manuscript outlines how some of the key forces operating at the state level, including penal cultures, institutional structures, and interest-group activism, conditioned state penal trajectories. Lastly, the piece identifies potential avenues for future state-level research on the forces shaping penal law and policy.