This literature review assesses the extent of age mixing and economic transactions in the sexual relationships of adolescent girls in sub-Saharan Africa and the behavioral dynamics of girls and men involved in these partnerships. The examination of more than 45 quantitative and qualitative studies finds that relationships with older partners and those that involve economic transactions are common and that these asymmetries are associated with unsafe sexual behaviors and increased risk of HIV infection. Although the reasons that adolescent girls engage in sexual relationships with older men are varied, receipt of financial benefits is a major motivation. The literature presents evidence that girls have considerable negotiating power over certain aspects of sexual relationships with older men, including partnership formation and continuation; however, they have little control over sexual practices within partnerships, including condom use and violence. The review discusses directions for further research and the implications of current knowledge for future interventions.