The world prison population is growing at a rate that exceeds the rate of population growth. This issue of Epidemiologic Reviews comprises articles in which researchers summarize what is known about some of the key health issues facing people in prison, particularly in relation to human immunodeficiency virus and other blood-borne viral infections. A key recurring theme is that addressing the health needs of people in prison is important to reducing health inequalities at the population level-that prisoner health is public health. The reviews also highlight some critical evidence gaps, notably the lack of evidence from low- and middle-income countries, and the limited number of longitudinal studies in which health behaviors, health outcomes, or health service experiences after release from prison are documented. Despite growing evidence of the poor health of detained adolescents, none of the included reviews considered this population. Further research on the health of young people who cycle through juvenile detention should be a priority. Despite a rapidly growing literature on the health of people who experience incarceration, some critical health issues remain poorly understood, and there has been insufficient attention devoted to co-occurring health conditions and the consequent need for coordinated care. Key populations in custodial settings remain understudied, limiting capacity to develop targeted, evidence-based responses to their health needs. The quality of many studies is suboptimal, and although rigorous, independent research in correctional settings can be challenging, it is not impossible and is critical to laying the groundwork for evidence-based reform.