Sexual partner concurrency, which fuels the spread of HIV, has been hypothesized as a cause of higher rates of HIV among low-income, urban African-Americans. Despite this hypothesis, little is known about the phenomenology of partner concurrency. To address this gap in the literature, we recruited 20 urban African-American men from a public STD clinic to elicit their ideas about partner concurrency. Five themes emerged during focus group discussions. First, there was a general consensus that it is normative to have more than one sexual partner. Second, men agreed it is acceptable for men to have concurrent partners, but disagreed about whether it was acceptable for women. Third, although men provided many reasons for concurrent partnerships, the most common reasons were that (a) multiple partners fulfill different needs, and (b) it is in a man's nature to have multiple partners. Fourth, men described some (but not all) of the negative consequences of having concurrent partners. Finally, men articulated spoken and unspoken rules that govern concurrent partnerships. These findings increase knowledge about urban, African-American men's attitudes toward concurrent partnerships, and can help to improve the efficacy of sexual risk-reduction interventions for this group of underserved men and their partners.