The following article is an attempt to evaluate the work of Hans M. Keller in the context of the now well-developed field of forest hydrology in Switzerland. lt describes the origin of this science and the consecutive contributions by the Swiss Federal Institute for Forest, Snow and Landscape Research founded in 1885 and of its forest engineers Arnold Engler (1869-1923), Hans Burger (1889-1973), Felix Richard (1915-1984) and Hans Keller (1936-1993). lt also mentions the events which resulted in the Federal Law of Forest Police in 1876 and the initiation of the first hydrological research basins shortly before the turn of the century. The closing remarks include the life history of Hans Keller and some of the author's reminiscences.