Barrier islands are sandy elongated islands or peninsulas that parallel the shoreline and are separated from the shore by lagoons or marshes. They are formed along coastlines, commonly along passive plate margins, where there is an abundant supply of source material and the tidal range is small enough so that the long shore currents and wave action are more important than the onshore-offshore flow of tidal currents. These types of islands can rise as much as 6 meters (20 feet) above sea level, and are 10 to 100 kilometers (6-60 miles) long and 1 to 5 kilometers (0.6-3 miles) wide. They are usually composed of a narrow beach 50 meters (165 feet) wide, and a broader zone of inland dunes that make up the most of the island.

Coasts with a tide range of less than 2 meters are known as microtidal coasts and the islands formed here are long and narrow. Shorter islands broken by numerous tidal channels, are found on mesotidal coasts, and have a tidal range of 2-4 meters. If the sediment supply is too low, the coastline will erode back. Macrotidal coasts are barrier-free and are influenced only by the tides.

Good examples have been studied along the Atlantic and Gulf Coastal Plain of North America. The islands migrate landward and seaward with changes in sea level and often comprise an important part of transgressive and regressive shorelines. Channels and spits of these islands are also prone to migration up and down the island.

These islands typically produce shoestring sands of high porosity and permeability within impermeable shale sequences; they become excellent reservoir rocks for petroleum. Barrier sands have also proven to be host rocks for uranium, and beach sands can be placer mined for gold, diamonds, and other heavy minerals. The marshes and lagoons behind the islands are important areas of coal accumulation.