This paper describes the relationship between psychiatry and epidemiology and the influence of Giel and his collaborators on its development in recent decennia. Between 1850 and 1950 the two disciplines differed in essential respects. Epidemiology dealt with large numbers of clearly defined and often acute cases, using a monocausal model, whereas psychiatry focussed on the individual patient whose vaguely described problems were multi-causally interpreted. Round 1950 the tide turned. As a result of its involvement with chronic disorders epidemiology had begun to think in multicausal terms. Pressed by rising psychopharmacology psychiatry had come to realise the importance of accurate description and classification. The necessary base for cooperation was laid. Since then Giel and his team have explored the whole range of topics in psychiatric epidemiology, be it at the cost of in-depth findings. Their work reflected the methodological state of the art and proved to be relevant scientifically and socially.