The U.S. Ocean Commission Report identified the need for regional ecosystem assessments to support coastal and ocean management. These assessments must provide greater understanding of physical and biological dynamics than assessments at global and national scales can provide but transcend state and local interests. This need and timeliness is apparent for Long Island Sound, where a multi-state regional restoration program is underway for America’s most urbanized estuary. Synthesis of the Long Island Sound ecosystem is needed to integrate knowledge across disciplines and provide insight into understanding and managing pressing issues, such as non-point sources of pollution, coastal development, global climatic change, and invasive species. Currently, there is a need for a comprehensive volume that summarizes the ecological and environmental dynamics and status of Long Island Sound and its myriad ecosystems. It has been 30 years since a comprehensive summary of Long Island Sound was prepared and 50 years since the pioneering work of Gordon Riley. Major advances in estuarine science are providing new insights into these systems, and yet, the condition of many estuaries is in decline in the face of continuing coastal development. There is an opportunity to lay a foundation for integrative coastal observing systems that truly provide the foundation for improved decision-making. This book will provide a key reference of our scientific understanding for work performed over the past three decades and guide future research and monitoring in a dynamic urbanized estuary.