In KwaZulu/Natal, South Africa, where HIV prevalence is among the world's highest, a longitudinal qualitative study of partnership dynamics and HIV preventive behaviors was conducted. 47 young adults aged 18-24 participated in in-depth interviews, and 29 were re-interviewed 2 years later. Five analytical domains emerged: primary partnerships, love and romance; secondary partnerships; pregnancy/parenthood; condom use/prevention; and contextual influences, including schooling and future aspirations. Primary relationships were long-lasting, with most men and women in the same relationship at 2-year follow-up. Secondary, casual partnerships were common for men and women, although these were shorter and changed frequently. Love and marriage aspirations were not viewed as incompatible with secondary partners. Condom use increased over time in some primary relationships, but decreased in others, and was nearly universal with non-primary partners. Pregnancy, school drop-out, and economic need strongly influence young people's lifecourse. These findings suggest the need to focus prevention efforts on the partnership context, including partner reduction, and structural factors that impede or enhance prevention success.