This book contains reviewed submissions from 60 percent of the presentations given at a special Symposium on the Cycling ofReduced Gases in the Hydrosphere as part of the 24th Congress of the International Association of Theoretical and Applied Limnology (SIL) held in Munich, Germany, August 13-19,1989. Because of the theme of this Congress, this Symposium deliberately focused on the aquatic environment, especially freshwater ecosystems. Most of the delivered papers dealt with general ecological processes and the cycling of methane and hydrogen, so it was agreed that additional chapters be solicited in the specific areas of nitrogen (N) and sulfur (S) gas cycling. Even though this resulted in a delay in publication, chapters were continually updated by authors during extensive editing and peer review. This greatly expanded book deals with most of the important reduced (and partially oxidized in the case of N20) gases and their hydrospheric/biospheric cycling. Information in these chapters should be of considerable interest and importance to the scientific community because many of the reduced gases are closely linked to the phenomenon of climate change. As the planet enters an uncertain future it is essential to develop an understanding about the formation and consumption processes of reduced (and partially oxidized) gases as well as their cycling pathways in the ecosphere. It is the desire of the editors that this book will contribute to such an understanding.