Systems Analysis and Simulation in Ecology, Volume III, and its companion, Volume IV, grew out of a symposium, Modeling and Analysis of Ecosystems, held at the University of Georgia, 1-3 March 1973. The purposes of the meeting were to (i) review the status of ecosystem modeling, simulation, and analysis; (ii) provide a forum for interaction between U.S. International Biological Program (IBP) Biome modeling programs and selected non-IBP investigations involving systems approaches to ecosystem analysis; and (iii) identify and promote dialogue on key issues in macrosystem modeling. The volume is organized into two parts. Part I treats ecosystem modeling in the U.S. IBP. The introductory chapter is followed by five chapters describing grassland, deciduous forest, desert, tundra, and coniferous forest biome modeling. The concluding chapter is one of critique and evaluation. Part II is devoted mainly to freshwater ecosystems, grading into the estuarine system in the last chapter. The five chapters of this section encompass a simple thermal ecosystem, small woodland streams, a reservoir, one of the Great Lakes, a lake reclaimed from eutrophication, and a major estuary under stress of human impact.