Scientists spent more than 15 years studying the physical and chemical characteristics and biological productivity of the estuary and documenting the abundance, distribution, and life histories of the major fish species. New sampling equipment and analytical methods were developed. Sophisticated models of the striped bass and other importanat Hudson River fish populations were developed. Most of the results of these studies are buried in contractor reports, magnetic tape libraries, and hearings transcripts. Our objective in producing this monograph is to make this information accessible to the scientific community. Our intent is to 1) summarize the scientific issues and approaches; 2) Present the significant results of the Hudson River biological studies; 3) describe the role of the studies in the decision-making process; 4) evaluate the successes and failures of the studies, and 5) present recommendations for future estuarine impact assessments.