Greece has an extremely large number of islands, with estimates ranging from somewhere around 1,200 to 6,000, depending on the minimum size to take into account. The number of inhabited islands is variously cited as between 166 and 227.

The largest Greek island by area is Crete, located at the southern edge of the Aegean Sea. The second largest island is Euboea, which is separated from the mainland by the 60m-wide Euripus Strait, and is administered as part of the Central Greece region. After the third and fourth largest Greek Islands, Lesbos and Rhodes, the rest of the islands are two-thirds of the area of Rhodes, or smaller.

The Greek islands are traditionally grouped into the following clusters: The Argo-Saronic Islands in the Saronic gulf near Athens, the Cyclades, a large but dense collection occupying the central part of the Aegean Sea, the North Aegean islands, a loose grouping off the west coast of Turkey, the Dodecanese, another loose collection in the southeast between Crete and Turkey, the Sporades, a small tight group off the coast of Euboea, and the Ionian Islands, located to the west of the mainland in the Ionian Sea (one of these islands, Kythira, is off the southern tip of the Peloponnese Peninsula and part of the Attica region, but still considered part of the Ionian Islands, mainly because of historical reasons). There are also many islands, islets and rocks that surround the coast of Crete.