ObjectivePeople with mental illnesses are understood to be overrepresented in the U.S. criminal justice system, and accurate prevalence estimates in corrections settings are crucial for planning and implementing preventive and diversionary policies and programs. Despite consistent scholarly attention to mental illness in corrections facilities, only two federal self-report surveys are typically cited, and they may not represent the extent of relevant data. This systematic review was conducted to develop a broader picture of mental illness prevalence in U.S. state prisons and to identify methodological challenges to obtaining accurate and consistent estimates.MethodsMEDLINE, PsycINFO, the National Criminal Justice Reference Service, Social Services Abstracts, Social Work Abstracts, and Sociological Abstracts were searched. Studies were included if they were published between 1989 and 2013, focused on U.S. state prisons, reported prevalence of diagnoses and symptoms of DSM axis I disorders, and identified screening and assessment strategies.ResultsTwenty-eight articles met inclusion criteria. Estimates of current and lifetime prevalence of mental illnesses varied widely; however, the range of prevalence estimates for particular disorders was much greater— and tended to be higher— in prisons than in community samples.ConclusionsDefinitions of mental illnesses, sampling strategies, and case ascertainment strategies likely contributed to inconsistency in findings. Other reasons for study heterogeneity are discussed, and implications for public health are explored.