The understanding of the dynamic nature of our global fresh water resource is a challenge that the United Nations Environment Programme (UNEP) and the World Health Organization (WHO), in cooperation with UNESCO and WMO, address through the water quality component of the Global Environment Monitoring System (GEMS/WATER). The dynamic nature of these resources reflects the continuous interplay between the natural processes of geochemistry, hydrology, sedimentology and those activities associated with the human occupation of river basins.The Water Quality of World River Basins selectively summarizes data contributed by countries participating in the GEMS/WATER Programme over the period 1976-90. The book also includes information gathered from many other sources. It summarizes the results of analysis and interpretation for 82 major river basins around the world. Some of these river basins are located in highly populated and industrialized areas where human impacts can be clearly seen. Other watersheds represent more pristine areas that are currently not under extensive human stress.This publication contributes to the scientific investigation of global water resources. As global population increases and our demands on fresh water supplies grow, economic and development activities continue to increase the stress on natural ecosystems. Using examples drawn from the GEMS/WATER data bank, this volume demonstrates how natural processes interact with anthropogenic factors to create the observed water quality conditions.It is said that knowledge empowers the individual. One of the essential purposes of the UNEP/GEMS Environment Library series is to disseminate knowledge of environmental conditions to people around the world who are directly or indirectly affected by changes occurring in this global resource. It is my hope that this publication will contribute to our knowledge of the trends and conditions of water resources in diverse regions of our world.