This article explores the relationship between level of social integration and various aspects of health. A search of the literature published since the mid-1970s (under the MEDLINE key words, "social ties," "social network," "social isolation," "social environment") presented strong evidence that social integration leads to reduced mortality risks, and to a better state of mental health. The evidence on physical health outcomes is less conclusive. There is no consistent evidence that social integration affects the incidence of disease (at least for cardiovascular outcomes). However, social integration does appear to have a highly beneficial effect on post-myocardial infarction prognosis (functioning and longevity). A physiologic basis for these effects on health outcomes is also indicated by research demonstrating that both social isolation and nonsupportive social interactions can result in lower immune function and higher neuroendocrine and cardiovascular activity while socially supportive interactions have the opposite effects. In conclusion, available data suggest that, although social integration is generally associated with better health outcomes, the quality of existing ties also appears to influence the extent of such health benefits. Clearly, individuals' networks of social relationships represent dynamic and complex social systems that affect health outcomes.