This study used an event-based approach to understand condom use in a probability sample of 309 homeless youth recruited from service and street sites in Los Angeles County. Condom use was significantly less likely when hard drug use preceded sex, the relationship was serious, the partners talked about “pulling out”, or sex occurred in a non-private place (and marginally less likely when heavier drinking preceded sex, or the partnership was monogamous or abusive). Condom use was significantly more likely when the youth held positive condom attitudes or were concerned about pregnancy, the partners talked about condom use, and the partners met up by chance. This study extends previous work by simultaneously examining a broad range of individual, relationship, and contexual factors that may play a role in condom use. Results identify a number of actionable targets for programs aimed at reducing HIV/STI transmission and pregnancy risk among homeless youth.