In the past three decades, incarceration has become an increasingly powerful force for reproducing and reinforcing social inequalities. A new wave of sociological research details the contemporary experiment with mass incarceration in the United States and its attendant effects on social stratification. This review first describes the scope of imprisonment and the process of selection into prison. It then considers the implications of the prison boom for understanding inequalities in the labor market, educational attainment, health, families, and the intergenerational transmission of inequality. Social researchers have long understood selection into prison as a reflection of existing stratification processes. Today, research attention has shifted to the role of punishment in generating these inequalities.