Classification processes determine the entire nature of the prison experience, including but not limited to where an inmate is housed, whom he or she is housed with, the programs and work assignments available to or required of an inmate, visitation rights, and the amount or type of movement an inmate has within a facility. In making these important decisions, prisons have long relied on procedures to determine where and how to supervise inmates under their control. These classification processes are designed to maintain prison order, allocate necessary treatment services, and provide a transparent basis for decisions that affect inmate life. Classification decisions can be divided into several types: initial security/facility classification and custody classification (determined by external classification systems), housing program and work assignments (determined by internal classification systems), and re-classifications. Inmates undergo a rigorous classification process upon reception into a corrections department’s custody and are often subject to re-evaluations throughout their time incarcerated. The predominant goal of classification systems is to identify and manage inmate risk, and many studies have focused on classification systems and inmate misconduct, recidivism, and violence. As classification systems are also intended to diagnose inmate needs and provide treatment, and ensure the efficient allocation of correctional resources, scholars have focused classification systems’ impact on adjustment to prison, provision of treatment, and prison crowding. Research has questioned the ability of a single classification system to assess the risk and need of all inmates. Many studies have considered gender equity in classification decisions as well as the additional treatment needs and management considerations for incarcerated individuals with mental illness. Additional research has focused on administrative segregation, or classification decisions that result in the isolation of those who cannot be housed in the general inmate population. Complicating this literature is the changing nature of classification systems over time and the fact that all states and the federal government have adopted unique classification systems. It is therefore difficult to draw conclusions about the state of knowledge on classification practices as a whole. Nevertheless, the past several decades have seen an emphasis on objective classification, or empirically based strategies to make classification decisions. The use of risk assessment tools has become commonplace in this realm. Though past research focused on classification systems as a means to control and monitor inmates, a promising future direction lies in needs assessments and the ability to deliver individualized treatment to incarcerated individuals.