Evidence-based approaches are assuming prominence in many health-care fields. The core ideas of evidence-based health care derive from clinical epidemiology and general internal medicine. The concept of evidence has yet to be analysed systematically; what counts as evidence may vary across disciplines. Furthermore, the contribution of the social sciences, particularly qualitative methodology, has received scant attention. This paper outlines a model of evidence that describes four distinct but related types of evidence: qualitative-personal; qualitative-general; quantitative-general and quantitative-personal. The rationale for these distinctions and the implications of these for a theory of evidence are discussed.