In the emerging field of implementation science, measuring the extent to which a new or modified healthcare program or practice is successfully implemented following an intervention is a critical component in understanding how evidence-based treatments become part of regular practice. This article reports on an evaluation of a nine-site project that developed and directed an organization-level intervention designed to implement improvements in preventing, detecting, and treating HIV for persons under correctional supervision. Using semi-structured interviews to elicit perceptions from Senior Researchers and Executive Sponsors at each of the nine sites, this article presents their views and observations regarding the success of the experimental intervention in their criminal justice setting. Within the areas of focus for implementation (either HIV prevention, testing, or linkage to community treatment) the complexity of programmatic needs was influential in determining perceptions of success. An organization's pre-existing characteristics, staffing, funding, and interorganizational relationships contributed to either the ease or difficulty of programmatic implementation. Results are discussed regarding furthering understanding of why new or modified healthcare interventions achieve success, including whether the intervention is a modification of existing practice or is a new intervention, and the choice of implementation strategy. (Publisher abstract modified)