Although jails are both common and consequential, affecting millions of individuals annually, they are a relatively understudied aspect of the criminal justice system. In this review, we first document the prevalence of jail incarceration, highlighting how jail incarceration has risen in tandem with the more commonly examined prison incarceration. Next, we describe the population of individuals in jail, paying particular attention to the heterogeneous and disadvantaged nature of this population. We document how jail incarceration is measured, demographically and in household surveys, and argue that jail incarceration has lasting and profound consequences for individuals, families, and communities. We conclude the review by suggesting directions for future research. Given the common nature of jail incarceration—in conjunction with the fact that jail incarceration creates, sustains, and perpetuates inequality—a better understanding of the prevalence, correlates, and consequences of jail incarceration is critical for fully understanding the link between the criminal justice system and inequality.