The four counties of Long Island (fig. 1) are underlain by a wedge-shaped sequence of unconsolidated deposits of Late Cretaceous and Pleistocene age that lie unconformably on crystalline bedrock (fig. 2). A saprolitic (weathered bedrock) zone 20 to 100 ft thick overlies the bedrock in most areas. The sequence of unconsolidated deposits thickens to the south and southeast by about 65 to 100 feet per mile and contains Long Island's fresh ground water. Long Island's ground-water system consists of four main aquifers-the upper glacial, the Jameco, the Magothy, and the Lloyd. The Lloyd aquifer underlies nearly all of Long Island (fig. 3), but pumpage from the Lloyd has been limited to the northern and southern coastal areas of the island by the New York State Department of Environmental Conservation since about 1955 (Garber, 1986). Coastal areas are exempt where the Lloyd is the only source of potable water. The former Jamaica Water Supply Corporation (now owned by New York City) is a noted exception withdrawing as much as 6 million gallons per day (Mgal/d) since the mid-1930s from the Lloyd in central Queens County. This paper: (1) provides a brief history of U.S. Geological Survey (USGS) studies that provided significant data on the Lloyd, (2) summarizes the hydraulic characteristics of the Lloyd as reported in those studies, and (3) describes present-day monitoring of the Lloyd by the USGS.