Epidemiologists and other researchers in the biomedical field have recently recognized the potential importance of social networks and the support they provide in maintaining health and well being. However, measuring social networks and social support has not proven to be easy, measures are generally not well validated and critical dimensions or characteristics of networks have yet to be identified. The aim of this paper is to provide the reader with some background on the work being done in this area. In particular, attention is devoted to why, historically, social networks have been suspected of playing a role in disease causation, what some important characteristics of networks are likely to be, examples of measures currently in use, and what the biological pathways are that may link networks to morbidity and mortality. A description of the Alameda County Study in California is given. In this study, people who were socially isolated were found to be over twice as likely to die in a nine-year follow-up period as those with many contacts. This increased risk was found to be independent of alcohol and cigarette consumption, obesity and other health practices.