In recent years, social networking sites have become a prevailing communication technology in the evolution of the digital era for today’s Internet users (Ipsos Insight 2007). As more and more marketers attempt to harness the power of electronic word-of-mouth (eWOM) in social networking sites (Williamson 2006), rigorous investigation of determinants that lead to consumers’ engagement in eWOM via the social networks is becoming critical. A central question to answer is what factors influence eWOM behavior in social networking sites and what are the underlying processes of eWOM communications in this new social medium. This study focuses on five social relationship variables: social capital, tie strength, homophily, trust, and consumer susceptibility to interpersonal influence that are all related to eWOM behavior in social networking sites. An online survey with a sample drawn from a large southwestern university was conducted to examine predictors of eWOM in social networking sites. Results from a series of multiple regression analyses indicate that certain social relationship variables are significant predictors that relate to social networking site users’ eWOM behavior. Out of the five relationship variables, social capital, homophily, trust, and interpersonal influence were found to significantly relate to users’ engagement in eWOM communications, whereas no effect was found with regard to tie strength. My dissertation research provides a theoretical understanding of consumers’ use of social networking sites as a vehicle for eWOM and contributes to the literature on computer-mediated communication with specific emphasis on online social media. Managerially, findings from this research could provide marketers with valuable information to establish their long-term relationships with consumers and use beneficial eWOM to promote selected brands. In conclusion, examining social relationships in social networking sites could contribute to our understanding of the determinants of consumer engagement in eWOM, which in turn influences the extent and pattern of eWOM and enables companies to deliberate their product diffusion strategies.