Small catchments provide attractive opportunities for biogeochemical research, as they encompass a number of interacting components: atmosphere and vegetation, plants and soils, bedrock and groundwater, surface waters and surrounding land, in an easily recognizable natural topographic boundary. This text presents data gathered from northern temperate regions, with a special chapter on studies in tropical countries. The main focus is on the fluxes of chemical elements both across the boundary of the catchment and within the catchment itself. The wide array of processes covered makes it a valuable tool for understanding the mechanisms of natural biogeochemical processes and their modification by human activity.