By starting with several requirements for the study of small natural hydrological catchment systems in a changing world, and with compiling important international environmental research activities with main focus on Europe it is shown that runoff formation is an ecohydrological key process which is still not very well known. This fact is confirmed by demonstrating common traditional runoff formation concepts and selected results from modern analytical (environmental tracer) approaches of understanding and adequately modelling runoff formation on a physical basis. Selected results are taken from several study basins of 1-10 km² in Germany, thus representing different orography (hilly lowland, mountainous highland), hydrologeology (porous and fractured rock aquifers) and land use (forestry, agriculture) conditions. Runoff formation which can be divided into the sub-processes of (i) infiltration with initial saturation, (ii) areal rise of groundwater table and pressure transmission, and (iii) groundwater exfiltration into river reaches is found to constitute a hydraulically similar physical process at least for this small basin scale regardless of mentioned environmental conditions, and groundwater recharge as the compensating process to be permanent. Finally, some major demands for future research activities are developed considering also practical water quality and management aspects.