Long Island is in southeastern New York, United States, separated from the rest of the state by the East River and from Connecticut by Long Island Sound. Long Island contains four counties, of which the western two are boroughs (Queens and Brooklyn) of New York City, and the other two (Nassau and Suffolk) are mainly suburban.

Long Island, as part of the Outer Lands region, is formed largely of four spines of glacial moraine, with a large, sandy outwash plain towards its barrier islands and the Atlantic Ocean. These moraines consist of gravel and loose rock left behind during the two most recent pulses of Wisconsin glaciation some 21,000 years ago (19,000 BC). The northern moraine, which directly abuts the North Shore of Long Island at points, is known as the Harbor Hill moraine. The more southerly moraine, known as the Ronkonkoma moraine, forms the "backbone" of Long Island; it runs primarily through the very center of Long Island, roughly coinciding with the length of the Long Island Expressway.

The land to the south of this moraine to the South Shore is the outwash plain of the last glacier. Part of this, known as the Hempstead Plains, supported one of the few natural prairies to exist east of the Appalachian Mountains.^[1]

The glaciers melted and receded to the north, resulting in the difference between the North Shore beaches and the South Shore beaches. The North Shore beaches are rocky from the remaining glacial debris, while the South Shore's are crisp, clear, outwash sand. Running along the center of the island like a spine is the moraine left by the glaciers. The glaciers also formed Lake Ronkonkoma, a kettle lake.

The island's tallest natural point is Jayne's Hill near Melville, with an elevation of 400.9 feet (122.2 m) above sea level. Long Island is separated from the mainland by the East River, not in fact a river, but a tidal strait. Long Island Sound forms the northern boundary of the island.

Long Island contains a series of sand and gravel aquifers, geologic formations which can hold, transmit, and yield water in usable quantities. All of Long Island's water supply comes from underground water reserves held in aquifers. Stacked one on top of the other like layers in a cake, three major and one minor aquifer make up the Long Island aquifer system. In sequence from shallowest to the deepest, the Long Island aquifers are: the Upper Glacial, the Magothy and the Lloyd Aquifers. All Long Island aquifers receive their fresh water from precipitation which takes from 25 to 1,000 years to migrate through the layers to the aquifers, which hold roughly 70 trillion US gallons (260 km³)—enough to flood the entire surface of Long Island with more than 300 feet (90 m) of water—and can withstand long droughts that could dry up surface-water reservoirs like the ones that supply New York City. Almost four million gallons each day are taken from beneath Nassau and Suffolk Counties, providing the primary source of water for the resident population. While most homes are on a municipal water system, there are still many areas where homes have their own wells to provide water. Both Nassau and Suffolk counties have long recognized their dependence on the aquifers and have stipulated that recharge basins (known locally as sumps^[2]) be built to collect ground water. Recharge basins are required and sized based upon the scale of any new development on Long Island. Due to contamination associated with development, concern to preserve the quality of Long Island's groundwater has become the single most important factor limiting the region's growth.

In United States v. Maine (1985), the U.S. Supreme Court treated the island as a peninsula for the purposes of a boundary decision.^[3] Despite the legal decision, the United States Board on Geographic Names considers Long Island an island, because it is surrounded by water.^[4]

Long Island has a climate similar to other coastal areas in the Northeast; it has hot and humid summers and cool to cold winters. The Atlantic Ocean helps bring afternoon sea breezes to the immediate south shore that temper the heat in the warmer months. Severe thunderstorms are common, nevertheless, especially when they approach the island from the mainland areas of the Bronx, Westchester County and Connecticut in the northwest. The humid subtropical climate zone (Cfa) according to the Köppen climate classification has expanded to include the majority of the area of Long Island (including Brooklyn, Queens, most of Nassau, and parts of Suffolk) due to global warming, This area according to the Trewartha climate classification is oceanic (Do.) Trewartha requires at least eight months to be at least 50°F to be considered subtropical and Long Island has six to seven such months. The remainder of the island has January average below freezing and a hot-summer humid continental climate (Dfa.) The hardiness zones are 7a and 7b.

• Geography of New York
• Geography of New York City
• Shelter Rock (North Hills, New York)