Over the past several years, prisoner reentry has emerged as an important policy issue on the public agenda. Soaring correctional expenditures are leading State legislators to examine whether reentry programming can slow the proverbial revolving prison door. In 2002, the Federal Government issued $100 million in grants to States to fund prisoner reentry initiatives over the next 3 years. This paper shows the challenges to implementing a prisoner reentry program by examining in detail the design and execution of the Offender Reentry Program (ORP) at the Suffolk County House of Correction, a local correctional facility in Boston. The analysis found that implementation challenges still arise even when programs are delivering measurable and statistically significant program effects and are well designed, well funded, and well administered. In addition, the researchers found that the internal capacity of the correctional facility and the external authorizing environment defined by statutes, administrative regulations, policymakers, and other agencies can substantially compromise overall program effectiveness. Specifically, in the case of ORP, the internal and external constraints have led to continued under-enrollment in the ORP, despite the large number of prisoners released from the facility each day who could benefit from the program. Four propositions about prisoner reentry that were learned from the analysis are discussed and include: correctional institutions do not have strong incentives to offer reentry programs; the external environment drives the available options; institutional operations and policies often get in the way of reentry in correctional settings; and while analysts and practitioners agree about the need for reentry programs, there is no clear consensus on reentry treatment models or the rank ordering of inmates to participate in such programs. The challenges to prisoner reentry cover a broad range of political, structural, organizational, and programmatic issues. Appendix, tables, figures, endnotes, and references