Protection strategies for the prevention of both unintended pregnancy and sexually transmitted infections (STIs) have become a focus of research interest in light of recent statistics on disease transmission and unintended pregnancy rates among young adults. Although the same sexual behavior leads to both, the methods most effective in preventing pregnancy and those most protective against STIs, including HIV, are not the same. Moreover, despite the fact that young adulthood is characterized by multiple sequential and concurrent relationships and decisions related to condom use are likely made with regard to a specific sexual partner, little research has addressed if and how relationship and partner factors influence the decision to use a method for disease prevention, pregnancy prevention or both disease and pregnancy prevention. Previous research has focused mainly on individual-level explanatory factors, included only a limited number of factors related to partners and relationship dynamics, and has relied on data from cross-sectional or retrospective surveys. Further, most studies have been conducted with women only. We propose to analyze secondary data from a racially/ethnically diverse sample of women and men aged 18-30 included in the Project on Partner Dynamics (POPD). The main objective of this application is to further our understanding of HIV acquisition/transmission by increasing knowledge regarding the intersection between reproductive behavior and HIV prevention. More specifically, we will examine choice and use of methods for pregnancy and disease prevention within the con- text of heterosexual relationships among a sample of young adult women and men at increased risk for HIV acquisition/ transmission. The specific aims of the study are to 1) investigate which partner-specific individual factors (e.g., perceived vulnerability to pregnancy and HIV/STIs, condom use self-efficacy) and relationship factors (e.g., commitment, relationship type, perceived exclusivity, and decision-making related to preventing pregnancy and disease) are associated with prevention method choice and consistency of use, and examine what factors predict change in method use over time and within partnerships; and 2) determine which partner- specific individual factors and relationship factors predict motivation to use condoms for disease prevention, pregnancy prevention or dual protection, and examine what factors predict change in motivation for condom use over time and within relationships. The POPD study includes data from 537 young adults recruited from the Los Angeles area who were interviewed 4 times, at 4-month intervals, over a 1-year study period. At each interview, participants provided detailed information about all heterosexual partnerships since the previous interview. Multilevel multinomial modeling will be used to investigate the associations between individual and relationship factors and the type and consistency of methods used for disease and pregnancy prevention. Findings will inform the design of clinical counseling and other programs to increase dual protection and prevent the acquisition/transmission of HIV/STIs and unintended pregnancy in this vulnerable population.