In this review, I examine explanations for why the United States is a world leader in its use of imprisonment. I first outline cross-national trends in incarceration and then evaluate the state of the literature and empirical evidence for why the United States is more punitive than other advanced industrialized nations. I argue a confluence of political, economic, and social factors distinct to the United States context are implicated in the punitive turn in the 1970s. Specifically, United States’ penal exceptionalism is the result of (1) a shift of criminal justice policy from the judicial to the legislative branch of government; (2) political responses to social and economic changes including deindustrialization and the upheaval of race relations in the 1960s; and (3) a weak welfare state. These changes stand in stark contrast to the dynamics of criminal justice policymaking among comparable nations. However, there is a need for more comparative research on the topic, as understanding the mechanism behind the stabilization or decrease of penal populations in other countries may better elucidate the reasons for America’s divergence from international trends.