Background : In Baja California, Mexico, high risk populations of male clients of female sex workers (FSWs) are at risk for acquiring HIV and experiencing harms related to substance use, while residents of rural communities are susceptible to the harms of drug-related gang violence. Nevertheless, factors related to HIV-related risk behavior, drug use, and gang violence remain under-studied in these vulnerable populations. Aims : The aims of this dissertation were : (1) to examine two methods of measuring unprotected sex among male clients of female sex workers in Tijuana, Mexico; (2) to examine the correlates of self-efficacy for condom use among male clients of female sex workers in Tijuana, Mexico; and (3) to examine the association between drug-scene familiarity and exposure to gang violence among residents in a rural farming community in Baja California, Mexico. Methods : In Chapter 2, 394 male clients of FSWs in Tijuana, Mexico were interviewed, and the results of measuring unprotected sex using dichotomous and count dependent variable approaches were compared. Chapter 3 examined quantitative survey data from Tijuana to identify socio-cognitive factors associated with self efficacy for condom use among 393 male clients. In Chapter 4, 169 residents in a small, rural community in Baja California completed surveys collecting information on drugs and gang violence. Results : In Chapter 2, the results of modeling the two analytic approaches measuring unprotected sex among male clients of FSWs showed that each approach identified several unique correlates that were not identified by the other. Chapter 3 identified psychosocial and psychosexual factors that may influence self-efficacy for condom use among male clients of FSWs. Chapter 4 found that exposure to gang violence was common and was associated with drug scene familiarity, suggesting a close relationship between drugs and gang violence in this community. Conclusions : Results from these preliminary analyses have important implications for the development of interventions to increase self-efficacy for condom use in order to influence safer sex among male clients of FSWs in Tijuana, and for interventions to prevent drug- and violence-related harms in rural communities in Baja California.