This paper considers how the Supreme Court of the United States (SC) and the European Court of Human Rights (ECtHR) apply, interpret and frame abstract imprisonment purposes, and how they view their relevance to prison conditions, while discussing the constitutionality of prison conditions. The paper argues that the SC and the ECtHR view, conceptualise and interpret the purposes of imprisonment differently. Regarding the purposes of retribution and rehabilitation specifically, the analysis presented in the paper exposes a ‘Janus face’, meaning that each purpose can, and is, interpreted in two different, and almost contrasting ways. The paper offers three themes regarding the conceptualisation of imprisonment purposes by the SC and the ECtHR: First, the relationship between the purposes of sentencing and imprisonment along the penal continuum, and the role of rehabilitation in a prison regime: should sentencing purposes be relatively static during their implementation in prison, meaning that retributive-oriented sentencing purposes should be pursued (SC), or should they conversely progress with the passage of time, from retribution to resocialization as the primary purpose of imprisonment (ECtHR). Second, the meaning of retributivism in regard to prison conditions: should prisoners pay a debt to society by suffering in restrictive prison conditions (SC), or is retributivism achieved by atonement and by finding ways to compensate or repair harms caused by crime (ECtHR). Third, the way in which prison rehabilitation is framed and understood: should prison rehabilitation be seen as a risk management tool aimed purely at lowering recidivism (SC), or as a moral concept grounded in a prisoner’s ability to change his life and belief in personal responsibility for one’s actions (ECtHR). Possible theoretical implications and general policy implications are considered in the paper.