This article summarises the importance of epidemiology to clinicians and aims to show how an understanding of epidemiological concepts can make an important contribution to optimum clinical practice in its broadest sense. Epidemiological principles can be applied to clinical practice in interpreting the results of diagnostic tests, assessing and communicating risk and prognosis, and in identifying appropriate treatment for individual patients. They are also of value to clinicians involved in planning, monitoring and improving services, teaching medical students and postgraduates, critically appraising medical literature, and undertaking or supervising research.