Hans Keller initiated forest hydrology research in Switzerland with the construction of the first hydrologic and climatic measuring station in the Alptal (Canton Schwyz) in 1968. During the past 25 years hydrological input and output data have been continuously gathered in this small catchment. In the beginning, the primary research goal was to determine the balance between rainfall and runoff in forested watersheds. Several years later, the water quality of rainfall, runoff and groundwater were additionally monitored. In the 1980's research was undertaken to study the transport of sediments in mountain torrents. Today, in order to investigate qualitative, quantitative and chemical changes in hydrological systems, long-term observations are of increasing importance. The analysis of the long-term data is presented with the help of three examples.