A crannog (/ˈkrænəɡ/; Irish: crannóg [ˈkɾˠan̪ˠoːɡ]; Scottish Gaelic: crannag) is typically a partially or entirely artificial island, usually built in lakes, rivers and estuarine waters of Scotland and Ireland. Unlike the prehistoric pile dwellings around the Alps which were built on the shores and were only inundated later on, crannogs were built in the water, thus forming artificial islands.

Crannogs were used as dwellings over five millennia from the European Neolithic Period, to as late as the 17th/early 18th century although in Scotland, convincing evidence for Early and Middle Bronze Age or Norse Period use is not currently present in the archaeological record. The earliest radiocarbon determinations obtained from key sites such as Oakbank in Loch Tay or Redcastle, Beauly Firth approach the Late Bronze Age - Early Iron Age transition at their widest interpretation at 2 sigma or 95.4% probability, falling after c.800BC and therefore could only be considered Late Bronze Age by the narrowest of margins. Crannogs have been variously interpreted as free-standing wooden structures, as at Loch Tay, although more commonly they exist as brush, stone or timber mounds which can be revetted with timber piles. However, in areas such as the Western Isles of Scotland, timber was unavailable from the Neolithic onwards. As a result, completely stone crannogs supporting drystone architecture are common here. Today, crannogs typically appear as small, circular islets, often 10 to 30 metres (30 to 100 ft) in diameter, covered in dense vegetation due to their inaccessibility to grazing livestock.