Social capital has become a popular subject in the literature on determinants of health. The concept of social capital has been used in the sociological, political science, and economic development literatures, as well as in the health inequalities literature. Analysis of its use in the health inequalities literature suggests that each theoretical tradition has conceptualized social capital differently. Health researchers have employed a wide range of social capital measures, borrowing from several theoretical traditions. Given the wide variation in these measures and an apparent lack of consistent theoretical or empirical justification for their use, conclusions about the likely role of "social capital" on population health may be overstated or even misleading. Elements of a research agenda are proposed to further elucidate the potential role of factors currently subsumed under the rubric of "social capital."