The objectives of this paper are to (1) understand the nature of men's extramarital sexuality in three low income communities in Mumbai, India; (2) explore the associations between marital relationships and extramarital sex; and (3) assess the implications of the research results for intervention. Results are based on survey data collected from 2,408 randomly selected men from the three study communities and a matched subset of 260 randomly selected men and their wives who responded to a female version of the men's survey. These surveys produced a unique data set, which allows sociodemographic, attitudinal and behavioral variables from husband and wife and variables that are the product of husband and wife interaction to be utilized to predict men's extramarital sex through multiple sequential logistic regression analysis. Results indicate that men's extramarital sex is significantly associated with husband's and wife's age, wife's perception of domestic violence, husband's education and place of birth, husband's alcohol use, wife's willingness to engage in marital sex, and types of marital sexual acts. These results confirm the need to move from the individual to the couple as the unit of research and the need for intervention to reduce the risk of HIV/STI transmission within marriage both in India and internationally.