Background: Young adults ages 18-24 are disproportionally affected by unintended pregnancy and sexually transmitted diseases and infections (STD/I). The best protection against both pregnancy and STD/Is is dual contraceptive use: the concurrent use of a highly effective contraceptive method and a condom. Objectives: This dissertation aims to increase our understanding of the psychosocial constructs associated with contraceptive and condom use. This project: 1) examines differences in contraceptive and STD/I knowledge by sex and race, and its association with method use; 2) determines the association between relationship characteristics and dual use; and 3) uses the Theory of Triadic Influence to examine direct and indirect associations between sociocultural factors, interpersonal factors, biological factors, and dual use. Methods: Young adult college students ages 18-24 (N=4,196) were invited to complete a web-based, cross-sectional, sexual health survey in Fall 2018. Multivariable linear and logistic regression models were run to determine differences in contraceptive knowledge by sex and race and its association with effective method use (N=436), and differences in STD/I knowledge by sex and race and its association with condom use (N=414). Multiple logistic regression models were run to determine the association between relationship characteristics, pregnancy and condom attitudes, demographics, and dual use (N=463). Structural equation modeling (SEM) was used to assess the standardized direct and indirect associations of sociocultural, interpersonal, and biological factors and dual use (N=406). Results: Increased contraceptive knowledge was associated with 1.114 odds increase in effective method use (95% CI: 1.058, 1.172), but no association was found between STD/I knowledge and condom use (aOR=0.970, 95% CI: 0.940, 1.000). Adjusted for all relationship characteristics, one-unit increase in trust was associated with decreased odds of dual use (aOR=0.982; 95% CI 0.966, 0.998). In independent models, having sex with a casual date/acquaintance (aOR=3.149; 95% CI: 1.550-6.397) compared to a romantic partner was associated with increased odds of dual use. The hypothesized SEM measurement model had poor fit and was re-specified. The final model had moderate fit and explained 70% of the variance in overall dual use. Condom attitudes (β = 0.18) and partner commitment (β = -0.22) were significantly associated with dual use through intention. Intention was significantly associated with dual use (β = 0.84). Conclusions: Findings emphasize the influential nature of interpersonal and biological psychosocial constructs on method use behavior. Health programs that address partner influences on STD/I risk perceptions, method use intention, and method use behavior could be beneficial for young adults. / Epidemiology