island chain, Monroe and Miami-Dade counties, southern Florida, U.S. Composed of coral and limestone, the islands curve southwestward for about 220 miles (355 km) from Virginia Key in the Atlantic Ocean (just south of Miami Beach) to Loggerhead Key of the Dry Tortugas in the Gulf of Mexico (Mexico, Gulf of). Bodies of water between the keys and the mainland include Biscayne (Biscayne Bay) and Florida (Florida Bay) bays.

The keys were originally inhabited by such Native American peoples as the Calusa and Tequesta. The Spanish explorer Juan Ponce de León (Ponce de León, Juan) visited the area in 1513. The first permanent settlers arrived about 1822 and engaged in fishing and salvaging shipwrecks. Population growth and economic development in the archipelago have fluctuated over time, reaching one of the highest peaks in the 1890s. The greatest disaster to strike the keys was a hurricane in September 1935 that killed hundreds of people and caused widespread property damage.

The western terminus of the Florida Keys is sometimes considered to be Key West, the most populous and economically developed of the islands. The Overseas Highway, running from the mainland to Key West, connects all of the main islands and is one of the longest overwater roads in the world, with 42 bridges, including one 7-mile (11-km) span. Completed in 1938, the highway was built over the route of the Florida East Coast Railway, finished in 1912 by financier and developer Henry M. Flagler (Flagler, Henry M) and destroyed by the 1935 hurricane.

Largest of the keys is Key Largo, about 30 miles (50 km) long and formerly known for its plantations of key limes (used to make key lime pies). John Pennekamp Coral Reef State Park, which contains large living coral formations, is the first undersea park in the United States. It is some 25 miles (40 km) long and 3 miles (5 km) wide and lies along Key Largo's east coast. Islamorada, located mainly on Upper Matecumbe Key, has a monument to World War I veterans and victims of the 1935 hurricane. Long Key State Park is on Long Key, just southwest of Islamorada. The main town of the middle keys is Marathon, a centre of extensive resort development. Nearby is the Museum of Natural History of the Florida Keys and a dolphin research centre. Bahia Honda State Park, on Bahia Honda Key, features a large area of tropical palms and beach recreation facilities.

Many of the keys fall within the boundaries of three national parks. Biscayne National Park, a short distance south of Miami Beach, includes several of the northernmost keys, and most of the keys in Florida Bay are within Everglades National Park (Everglades). Dry Tortugas National Park, which includes historic Fort Jefferson (begun 1846), encompasses all the westernmost keys. The keys are protected by the Florida Keys National Marine Sanctuary, established in 1990, which covers an area of some 3,600 square miles (9,300 square km). Much of the northern area of the lower keys has been designated the Great White Heron National Wildlife Refuge, and another refuge is located immediately west of Key West. Big Pine Key, largest of the lower keys, is a refuge for the tiny key deer and has unusual displays of cacti.

The Florida Keys are home to a wide variety of plant and animal life. Mangroves, sea grasses, and coral reefs are abundant. Animals such as alligators, sea turtles, and the endangered manatee can be found there, and more than 600 species of fish live in the reefs. The keys are a popular tourist destination, and tourism and commercial fishing are the major components of the economy.