African Americans face disproportionately higher risks of HIV infection. Concurrent sexual relationships, or sexual partnerships that overlap in time, are more common among African Americans than individuals of other races and may contribute to racial disparities in HIV infection. However, little is known about attitudes, norms and practices among individuals engaged in concurrent partnerships. Little is also known about the processes through which structural, behavioral, and social factors influence concurrent sexual relationships. We recruited 24 heterosexual African American men involved in concurrent sexual relationships from a public health clinic in Philadelphia. We conducted in-depth interviews exploring these men's sexual practices; social norms and individual attitudes about concurrency; perceived sexual health risks with main and non-main partners; and the social, structural, and behavioral factors contributing to concurrent sexual relationships. Twenty-two men reported having one main and one or more non-main partners; two reported having no main partners. Respondents generally perceived sexual relationships with non-main partners as riskier than relationships with main partners and used condoms far less frequently with main than non-main partners. Most participants commented that it is acceptable and often expected for men and women to engage in concurrent sexual relationships. Social factors influencing participants' concurrent partnerships included being unmarried and trusting neither main nor non-main partners. Structural factors influencing concurrent partnerships included economic dependence on one or more women, incarceration, unstable housing, and unemployment. Several men commented that individual behavioral factors such as alcohol and cocaine use contributed to their concurrent sexual partnerships. Future research and interventions related to sexual concurrency should address social and structural factors in addition to conventional HIV risk-taking behaviors.