Lake Okeechobee is considered to be the «liquid heart» of the interconnected Kissiee River / Lake Okeechobee / Everglades ecosystem of south Florida, USA. This large (1732 square km) shallow lake (mean depth 2.7 m) is vital natural resource; it provides flood protection and water supply and serves as a recreation area for more than 3,5 million people. A diverse littoral region occupies 25 an important habitat for economically important fish and wildlife stocks, including several endangered species. Human activities in the latter part of this century have deteriorated the quality of the Lake Okeechobee ecosystem. Diffuse inputs of nutrients (mainly from cattle ranching and diary operations to the north) have doubled the phosphorus concentration of the lake water between 1970 and 1980. Excess nutrient loading has been the cause of an increased frequency and intensity of blue-green algal blooms -- a signal of lake eutrophication. Surface scums with unpleasant odors as an obvious sign of declining water quality have heightened public concerns and led to the formation of committees and advisory groups to provide recommendations for lake protection and recovery. The water quality monitoring programs were supplemented with a substantial research program. In the wake of the algal blooms in the 1980s several interdisciplinary research projects were initiated, including a study of the lake's phosphorus dynamics, a watershed phosphorus transport and retention study, and a study of the lake's ecosystem. The present volume presents the results of the Lake Okeechobee ecosystem study. The ecosystem study, along with the results of other studies, has provided a scientific foundation for better resource management decisions and has increased our understanding of Lake Okeechobee and subtropical lakes in general.