Four categories of modern seafloor sedimentary environments have been identified and mapped across the large, glaciated, topographically complex Long Island Sound estuary by means of an extensive regional set of sidescan sonographs, bottom samples, and video-camera observations and supplemental marine-geologic and modeled physical-oceanographic data. (1) Environments of erosion or nondeposition contain sediments which range from boulder fields to gravelly coarse-to-medium sands and appear on the sonographs either as patterns with isolated reflections (caused by outcrops of glacial drift and bedrock) or as patterns of strong backscatter (caused by coarse lag deposits). Areas of erosion or nondeposition were found across the rugged seafloor at the eastern entrance of the Sound and atop bathymetric highs and within constricted depressions in other parts of the basin. (2) Environments of bedload transport contain mostly coarse-to-fine sand with only small amounts of mud and are depicted by sonograph patterns of sand ribbons and sand waves. Areas of bedload transport were found primarily in the eastern Sound where bottom currents have sculptured the surface of a Holocene marine delta and are moving these sediments toward the WSW into the estuary. (3) Environments of sediment sorting and reworking comprise variable amounts of fine sand and mud and are characterized either by patterns of moderate backscatter or by patterns with patches of moderate-to-weak backscatter that reflect a combination of erosion and deposition. Areas of sediment sorting and reworking were found around the periphery of the zone of bedload transport in the eastern Sound and along the southern nearshore margin. They also are located atop low knolls, on the flanks of shoal complexes, and within segments of the axial depression in the western Sound. (4) Environments of deposition are blanketed by muds and muddy fine sands that produce patterns of uniformly weak backscatter. Depositional areas occupy broad areas of the basin floor in the western part of the Sound. The regional distribution of seafloor environments reflects fundamental differences in marine-geologic conditions between the eastern and western parts of the Sound. In the funnel-shaped eastern part, a gradient of strong tidal currents coupled with the net nontidal (estuarine) bottom drift produce a westward progression of environments ranging from erosion or nondeposition at the narrow entrance to the Sound, through an extensive area of bedload transport, to a peripheral zone of sediment sorting. In the generally broader western part of the Sound, a weak tidal-current regime combined with the production of particle aggregates by biologic or chemical processes, cause large areas of deposition that are locally interrupted by a patchy distribution of various other environments where the bottom currents are enhanced by and interact with the seafloor topography.