The purpose of this article is to determine whether the positive association between social support and well-being is attributable more to an overall beneficial effect of support (main-or direct-effect model) or to a process of support protecting persons from potentially adverse effects of stressful events (buffering model). The review of studies is organized according to (a) whether a measure assesses support structure or function, and (b) the degree of specificity (vs. globality) of the scale. By structure we mean simply the existence of relationships, and by function we mean the extent to which one’s interpersonal relationships provide particular resources. Special attention is paid to methodological characteristics that are requisite for a fair comparison of the models. The review concludes that there is evidence consistent with both models. Evidence for a buffering model is found when the social support measure assesses the perceived availability of interpersonal resources that are responsive to the needs elicited by stressful events. Evidence for a main effect model is found when the support measure assesses a person’s degree of integration in a large social network. Both conceptualizations of social support are correct in some respects, but each represents a different process through which social support may affect well-being. Implications of these conclusions for theories of social support processes and for the design of preventive interventions are discussed.