The opioid epidemic has resulted in significant increases in HIV, HCV, and syphilis among people who inject drugs. The intent of this Guide is to build the capacity of Opioid Treatment Programs (OTPs), with an emphasis on publicly funded OTPs, to integrate infectious disease prevention, screening, and treatment services. Integrating infectious disease services at OTPs has the potential to both (i) enhance prevention, screening, and treatment by addressing key gaps in patient care, and (ii) support patients’ recovery journeys by improving their overall well-being. This Guide provides insights, examples, and strategies to support OTPs in integrating infectious disease services into behavioral health programming.