The ecology of aquatic systems has developed most of its basic concepts and methods by studying small, homogenous and well delimited natural systems. Lakes, in this respect, are ideal objects of ecological study, and this is why the functioning of these systems has long been better understood than that of river systems. Moreover, the problem of lake eutrophication was a matter of concern as early as the 1950s, and prompted numerous studies (e.g. Vollenweider, 1968). Since then, there have been many cases of successful lake restoration. Understanding and obviating river alteration by human pressures represent a much bigger challenge. In some cases, such as organic pollution by point discharges of waste water, the alteration is local and concerns a limited stretch of river. As early as in 1925, with the work of Streeter and Phelps, the basic knowledge was acquired to understand and manage this kind of alteration. In most cases, however, the dimension of human impact on river systems is much greater. Hydraulic management (for navigation or flood and drought regulation), diffuse pollution by changing agricultural practices, landscape modifications, all alter the functioning of the whole drainage network in a way which is much more difficult to investigate scientifically. The need for basin-scale management of water resources is now recognized by most authorities. But, sometimes, the scientific basis is still insufficient to allow coherent actions to be decided at the basin and landscape scale, in order to preserve or restore biodiversity and water quality. This volume reflects the discussions along these lines that were held during the Symposium ("Man and River Systems") in Champs-sur-Marne (Paris) in March 1998, as the joint initiative of three large interdisciplinary Research Projects implemented on large river basins in Europe, namely the PIREN-Seine programme (CNRS, Seine Basin, Meybeck et al. - eds-, 1998), BINOCULARS (CE-DGXII, Seine, Loire, Scheldt, Rhine, Aliakmon, Exe, Severn basins), the ICPMS project (ICPMS & CE-DGXI, Mosel basin). The purpose of all these projects is to better describe the functioning of large river systems, by considering biological, chemical and hydrological processes at the scale of the watersheds, and to evaluate the influence of humans on these processes. The knowledge gained in the course of these projects and of other broad-scale investigations has now been collected and discussed in 32 papers, grouped into four main topics.