Santorini is a small, circular archipelago of volcanic islands located in the southern Aegean Sea, about 200 km (120 mi) southeast from Greece's mainland. The largest island is known as Thēra (or Thira, Greek Θήρα [ˈθira]), forming the southernmost member of the Cyclades group of islands, with an area of approximately 73 km2 (28 sq mi) and a 2001 census population of 13,670. It is composed of the Municipality of Thera (pop. 12,440) and the Community of Oía (Οία, pop. 1,230, which includes 268 inhabitants resident on the offshore island of Therassia, lying to the west).

These have a total land area of 90.623 km2 (34.990 sq mi), which also includes the uninhabited islands of Nea Kameni, Palaia Kameni, Aspronisi, and Christiana (all part of the Municipality of Thira).
Santorini is essentially what remains of an enormous volcanic explosion, destroying the earliest settlements on what was formerly a single island, and leading to the creation of the current geological caldera.
Santorini became independent from Ottoman rule in 1821, during the Greek War of Independence and was united with Greece in 1830 under the Treaty of London. In 1956 a major earthquake and a volcanic eruption resulted in the demolishing of many buildings on the island, leading to the desertation of many of its villages. Nowadays due to the expansion of tourism, the island has gained great wealth and population. Its major settlements include Fira , Oia, Emporio, Kamari, Perissa, Imerovigli, Pyrgos, and Therassia. Akrotiri is a major archaeological site, with ruins from the Minoan era. Santorini's primary industry is tourism, particularly in the summer months. The island's pumice quarries have been closed since 1986, in order to preserve the caldera.

The island is the remnant of a volcanic cone whose top was blown off. The inner coast around the caldera is a sheer precipice of more than 300 m drop at its highest, and exhibits the various layers of solidified lava on top of each other, and the main towns perched on the crest. The ground then slopes outwards and downwards towards the outer perimeter, and the outer beaches are smooth and shallow. Beach sand color depends on which geologic layer is exposed; there are beaches with sand or pebbles made of solidified lava of various colors: the Red Beach, the Black Beach, the White Beach, etc. The water at the darker colored beaches is significantly warmer because the lava acts as a heat absorber.

The traditional architecture of Santorini is similar to that of the other Cyclades, with low-lying cubical houses, made of local stone and whitewashed or limewashed with various volcanic ashes used as colors. The unique characteristic is the common utilisation of the hypóskapha: extensions of houses dug sideways or downwards into the surrounding pumice. These rooms are prized because of the high insulation provided by the air-filled pumice, and are used as living quarters of unique coolness in the summer and warmth in the winter, and also as premium storage space for produce and especially for wine cellaring: the Kánava wineries of Santorini

Santorini is one of the few Cyclades Islands with a major airport. It is about 6 km southeast of downtown Thira. The main asphalt runway (16L-34R) is 6972 feet in length. The parallel taxiway was built to runway specification (16R-34L). It can accommodate Boeing 757, Boeing 737, Airbus 320 series, Avro RJ, Fokker 70, and ATR 72. Scheduled airlines include Olympic
Air lines, Aegean Air lines and Athens Airways, with chartered flights from other airlines during the summer. Transportation to and from the air terminal is through buses, taxis, hotel car-pickups and rental cars.