Continuity of care is basic to the value system of family practice. Yet it is not often treated as a measurable quality, and the role of "continuity" in improving the "outcome" of health care is infrequently evaluated. "Continuity" can be understood in terms of its elements: continuity of a professional relationship, continuity of information, and the actual delivery of service or care. These elements in turn can be related to outcome by means of a series of assertions or hypotheses which together constitute a "theory" of continuity of care. This paper develops these ideas and reviews existing research which has examined continuity of care.