The Thousand Islands constitute an archipelago of 1,864 islands that straddles the Canada-U.S. border in the Saint Lawrence River as it emerges from the northeast corner of Lake Ontario. They stretch for about 50 miles (80 km) downstream from Kingston, Ontario. The Canadian islands are in the province of Ontario, the U.S. islands in the state of New York.

The 1,864 islands range in size from over 40 square miles (100 km2) to smaller islands occupied by a single residence, or uninhabited outcroppings of rocks that are only home to migratory waterfowl. To count as one of the Thousand Islands, emergent land within the river channel must have at least one square foot (0.093 m2) of land above water level year-round, and support at least two living trees.

In the late 19th and early 20th centuries, many distinguished visitors made the region widely known as a summer resort. Several grand hotels provided luxurious accommodations while steamboats offered extensive tours among the islands. Wealthy and middle-class summer residents built summer homes. Some masonry "castles" remain as international landmarks. The most famous extant examples are "The Towers" on Dark Island, now called Singer Castle, and the long-neglected Boldt Castle on Heart Island, much of which has been completed over the recent decades in accord with Mr. Boldt's original plans – hitherto, it had been left unfinished for over 75 years upon the untimely death of his wife. The original plans were updated to incorporate numerous current technological conveniences.