In the first section of this book an introduction is made to the methods used to decribe disease in populations. Measurement of the frequency and changing patterns of diseases is essential to ensure that the deploymnet of medical resources matches the community's needs. Without such measurements, needs for health care may be unidentified. The second section describes the application of epidemiology in the discovery of causes of disease. Most hypotheses about causation subsequently confirmed by epidemiological methods have arisen from the day-to-day experience of alert clinicians, and many more will do so in the future. In the past, doctors tended to regard the practical application of these discoveries to the prevention of disease as being outside their sphere of responsibility. However in obstetrics, paediatrics and general practice, prevention has become accepted as a normal part of medical practice, and concern for disease prevention is gaining ground in other branches of medicine. In the third section an account is given of four aspects of patient care which require an understanding of epidemology: screening, prognosis, epidemics, and the evaluation of medical services. The text is aimed at undergraduate students and practising clinicians (non-specialist epidemiologists).