Because individuals at risk for HIV and STDs are concentrated in prisons and jails, incarceration is an opportunity to provide HIV and STD testing. We interviewed 72 service providers working in U.S. prisons in four states about their experiences with and perceptions regarding HIV and STD testing in prison. Providers' job duties represented administration, education, security, counseling, and medical care. Providers' knowledge of prison procedures and programs related to HIV and STD testing was narrowly limited to their specific job duties, resulting in many missed opportunities for prevention counseling and referral. Suggestions include increasing health care and counseling staff so posttest counseling can be provided for those with negative as well as positive test results, providing additional prevention programs for incarcerated persons, improving staff training about HIV and STD testing, and improving communication among in-prison providers as well as between corrections and public health staff.