The article describes men's perceptions of and experience with substance use and sexual behavior during incarceration. Grounded theory content analyses were performed on qualitative interviews conducted with 80 men, aged 18 to 29, in four U.S. states. Participants believed that drugs were easily available in prison. Half reported using substances, primarily marijuana or alcohol, while incarcerated. Key themes included the role of correctional personnel in the flow of substances in prison and the economic significance of substance traffic king. With regard to sexual behavior, most men acknowledged that it occurred but were hesitant to talk in-depth about it. There was a strong belief in "don't look, don't tell," and sex in prison was often associated with homo sexual behavior or identity. Sex during incarceration was reported by 12 men, mostly with female partners. Participants were pessimistic about HIV/STD/hepatitis prevention efforts inside correctional facilities. These findings highlight the need for risk reduction programs for incarcerated men.