Certain sexual partnering practices, such as multiple, concurrent, or age-discrepant partnerships, are known to increase HIV risk. Yet the underlying dynamics of young people's relationships are less clearly understood. Using household survey and qualitative data, this study examines partnership dynamics and characteristics in the context of HIV risk, including number of partners, age differences, partnership duration and concurrency, and frequency of contact among young people aged 15-24 in rural KwaZulu-Natal, South Africa. One-third of the men surveyed reported multiple and/or concurrent partnering, and one-fourth of the women had partners who were five years older than they were. Nonparticipation in civic organizations or school was correlated with higher-risk partnerships for women but not for men. On average, relationships lasted more than a year for the women and men surveyed, and were frequently characterized as "serious." Qualitative findings pointed to the sequential and overlapping nature of relationships, however, with distance and mobility being important influences. These fluid partnership patterns are an important feature of young people's sexual risk in the context of South Africa's severe HIV epidemic.