The epidemiological method is indispensable to the clinical investigator, the practising clinician and the public health worker. In the field of mood disorders epidemiological inquiry has been hampered by the subjective nature of the phenomena under study, the problems of assessment and diagnosis, observer bias and the unrepresentative nature of small population samples. Nontheless, it is argued that the difficulties are similar to those encountered elsewhere in medicine and examples are given of epidemiological contributions to an understanding of depression.