Long Island Sound is one of the largest estuaries along the Atlantic coast of the United States. It is a glacially produced, semi-enclosed, northeast-southwest-trending embayment, which is 150 km long and 30 km across at its widest point. Its mean water depth is approximately 24 m. The eastern end of the Sound opens to the Atlantic Ocean through several large passages between islands, whereas the western end is connected to New York Harbor through a narrow tidal strait. Long Island Sound abuts the New York-Connecticut metropolitan area and contains more than 8 million people within its watershed. A study of the modern sedimentary environments on the sea floor within the Long Island Sound estuarine system was undertaken as part of a larger research program by the U.S. Geological Survey (Coastal and Marine Geology Program) conducted in cooperation with the State of Connecticut Department of Environmental Protection and the U.S. Environmental Protection Agency. Knowledge of the bottom sedimentary environments was needed to discern the long-term fate of wastes and contaminants that have been, or potentially will be, introduced into the system and to help understand the distribution of benthic biologic habitats.