Correctional classification is at the core of the prison experience. Classification processes determine what, with whom, and how an inmate will spend his or her time while incarcerated. Classification designation influences virtually all dimensions of prison life, including the structure of inmate routines, ability to move about the facility premises, program eligibility, mandatory treatments, and housing location or style. Yet it is very challenging to speak about correctional classification in general terms, because there are 51 different classification schemes in the United States, one for each of the 50 states and the federal prison system. Correctional classification can be centralized or decentralized to varying degrees across institution, facility, and unit levels of prisons. Although often used interchangeably in correctional argot, the two predominant correctional classification types are security (referring to the characteristics of the prison) and custody (referring to the permissions of the inmate). Classification structures and processes shape much of the prison experience and, as such, are central to investigations of the effects of prison on inmate outcomes. Indeed, the extent of the deprivations inmates face during incarceration is largely determined by their institution, facility, unit, and custody levels. Discussing correctional classification across systems is challenging because classification designations take on a heterogeneous, nested structure, meaning that in some systems institution and facility are the same, in other systems facility and custody are the same, and in still other systems institution, facility, and custody are all distinct, with custody nested in facilities nested in institutions. In addition to classification structures, there are classification processes which are the set of procedures that correctional administrators use to determine security and custody levels. Classification criteria, processes, and timelines vary across departments of corrections. The general goals of classification procedures are to minimize the probability of escape and maximize the security of the department facilities, inmates, and staff, while housing the inmate at the least restrictive level possible and providing appropriate services. Correctional administrators must balance security and rehabilitative concerns in custody and security classification practices. In what is often described as a direct trade-off, most agencies prioritize the security and safety of inmates and staff over the treatment needs of inmates.