This publication, part of SAMHSA's Evidence Based Resource Guide series, addresses the co-occurrence of HIV and mental illness and/or SUD. It reviews effective programs and practices to prevent HIV and, for those with HIV, to increase linkage and retention in care in order to improve health outcomes. It describes five categories of effective interventions including: Practices to increase uptake and adherence to Pre-Exposure Prophylaxis (PrEP); Syringe Services Programs (SSPs); Contingency Management (CM); Cognitive Behavioral Therapy (CBT); and Patient Navigation. The guide provides examples of how practitioners can use these practices in their programs, and resources for evaluation and quality improvement.