Participants will be recruited from diverse venues: OAT clinics, community health centers, syringe exchange programs, community-based organizations, homeless programs, and cohorts established by research studies. The clinical sites will determine eligibility based on clinical records, or on-site testing including for HCV tests (anti-HCV and HCV viremia) and drug toxicology testing as needed. Study participants will be screened, consented and enrolled on-site at OAT and non-OAT clinic settings.