Drug abuse and addiction are major burden on the society and a public health problem. Marginalized populations such as drug users are especially at a risk for several health problems, including HIV, Hepatitis B, C and drug overdose. The health risk of this population stems not only from human capital elements such as education, skills, family background and values but also from the dynamics emerging from within their social networks. Social capital is a multifaceted construct comprising of support and norms of trust, reciprocity and in the case of the present work, drug use. Based on a combined social capital theory given by Colman (1988) and Putnam (1993), the work presented in this dissertation aimed to 1) conduct a comprehensive literature review to assess the evidence (1980-2010) linking social capital and illicit drug use, and address the gaps in the current literature in studying the link between social capital and illicit drug use, 2) develop a quantitative measure of social capital as a latent construct among current injection drug users (IDU), and 3) prospectively assess the relationship between social capital and the risk behaviors associated with injection drug use. Data was used from Waves 1 and 2 of the SHIELD (Self-Help In Eliminating Life-threatening Diseases) HIV Intervention Study. Quantitative methods of factor analysis, structural equation modeling and multivariate logistic regression were utilized in the study to explore these relationships. A first-order factor analysis identified a single-factor model of social capital with indicators of social support, social participation and norms of trust and reciprocity. While there was no statistically significant relationship found between social capital and non-syringe paraphernalia sharing at follow-up, a significantly inverse relationship between social capital and sharing unclean needles at follow-up was found. Over all, the work presented in this dissertation presents the first examination of social capital as a latent construct in the context of illicit drug use in adult population and expands our understanding of how network based social capital can be leveraged to change risk behaviors and reduce the risk of disease transmission.