Prison privatization is the most significant development in penal policy in the second half of the 20th century. If it is not properly regulated and made rigorously accountable, privatization could have regressive effects. The challenge is to ensure that it is harnessed and driven for the benefit of imprisonment standards as a whole. The article reviews some of the main issues of principle and practice relevant to today's privatization: (1) prisoner mix; (2) spread across jurisdictions; (3) the change from management only contracts of early privatization arrangements to design, construct, finance and manage contracts; and (4) purchaser/provider split. The article also discusses costs/value for money, scrutiny of performance, accountability, standards, recidivism and cross-fertilization of private and public sector performance. Table, references