Crannogs are ancient artificial islands found in Scotland and Ireland, which typically had some sort of dwelling place constructed on them that served variously as farmers' homesteads, status symbols, refuges in times of trouble, hunting and fishing stations, etc. Substantial research has been carried out for similar sites in mainland Europe, which has demonstrated that they were lakeside settlements, mostly dating to the Neolithic period and not built over open water. In contrast, the Scottish and Irish sites were built in open water, clearly separate from the shore. In Perthshire, some prehistoric crannogs were originally timber-built roundhouses supported on piles or stilts driven into the loch bed. Today, these crannogs appear as tree-covered islands or remain hidden as submerged stony mounds. Until recently, there were few radiocarbon dates for these structures and so the sites appeared as a homogeneous group. Not only did this make it impossible to examine them in sub-groupings but it also inhibited research, as they did not fit into known periods or architecturally distinct sub-groups, except that they were surrounded by water. Recent work in Loch Tay has resulted in 14C dating of the timber piles from 13 of the 18 crannogs in the loch, allowing them to be fitted into different classes. A major group was constructed in the Early Iron Age around 400–800 BC, with smaller groups constructed around 200–300 BC and 0 BC/AD. There is also evidence of repair/reoccupation of some of these crannogs in the 6th–9th centuries AD. A number of the sites were also known to be inhabited into the recent past, with one, Priory Island, occupied until the 17th century. The dates of construction also raise important issues relating to the loch-level changes that have taken place. The 14C results will be discussed in relation to the periods of origin and habitation of the crannogs.