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.