In April 1992 the Foundation Eni Enrico Mattei organized a workshop on the regulation of nonpoint source pollution. This volume inc1udes the proceedings of that meeting, as well as additional original contributions, in an attempt to provide an overview of recent theoretical developments in the field. Research on the causes, consequences, and control of nonpoint source pol­ lution has been carried out over the last two decades. Interest in this subject has grown as a result of the increasing recognition of the insufficiency of traditional pollution control policies focused on the large scale, confined, and general­ ly predictable pollutant discharges. In fact, many contemporary problems are caused by the combined activities of small polluters, along with natural pro­ cesses, intermittent and unpredictable events, and often involve pollutants with complex environmental outcomes. Despite the progress made in understanding the nature and size of pollution from diffuse sources, the issue of regulation is still far from being system at­ ically and adequately addressed. This policy vacuum is partly attributable to the difficulty of adapting the traditional point source regulatory tool kit to the specific features of nonpoint source problems. Such features inc1ude the tech­ nical difficulty of identifying sources and measuring individual emissions, their variability over time and space, the role played by natural processes in detennin­ ing pollutant discharges at source and their ultimate impacts on the receiving environmental media.