Islands are areas of land surrounded by water, ranging from small rocky outcrops to large and varied landscapes. They occur in oceans, seas, lakes, and rivers. Although their isolation is an obvious feature, no island is completely separate from its surroundings. Water, air, weather, and the movement of organisms continually connect it with places beyond its shoreline.

Islands can have different origins. Some are the exposed tops of volcanic structures, while others are parts of a larger landmass separated by water. Sediment can also accumulate into islands in rivers or along coasts. These origins influence the shape of the land and the kinds of material found there. Later erosion and deposition continue to modify the shoreline, so an island's outline is not necessarily permanent.

The coast forms an important boundary between land and water. Waves act on exposed shores, while more sheltered bays may collect finer sediment. Tides can reveal and cover areas around the edge of an island. Inland conditions may change over surprisingly short distances, particularly where hills affect rainfall or shelter parts of the land from wind. A small island can therefore contain several distinct environments.

Isolation influences the arrival and survival of plants and animals. Some seeds travel by wind or water, and birds can cross stretches of open sea. Other organisms have more difficulty reaching a new island. Once established, populations interact with local resources and conditions. The distance from other land, the size of the island, and its history all help shape the community that develops there.

Human life on islands also reflects a balance between local resources and outside connections. Harbours, boats, roads, and settlements reveal how people move goods and reach different parts of the coast. Fresh water and sheltered ground can be especially important in choosing where to live. Considering both the island and the surrounding water gives a fuller understanding than treating the shoreline as the end of everything that matters. Changes in weather can make these outside connections easier on one day and much more difficult on another.