According to the Encyclopædia Britannica, an island is quite simply "any area of land smaller than a continent and entirely surrounded by water". This chapter reviews the 'state of the art' of urban island studies. Broadly speaking, island cities can be broken down into two primary categories: major population centres of larger island geographies; and densely urbanised small islands and archipelagos. An island population centre's density or absolute size is less important than its relative impact on, or fulfilment of, urban functions for its hinterlands. The largest and most densely populated cities in sub-Saharan Africa and the USA are based on small islands. Grydehøj identifies three primary benefits that small island spatiality lends to efforts to establish trading posts and/or centres of political power: territorial, defence and transport. Territorial benefits involve the political use of clear spatial demarcations associated with small islands to lay claim to a particular piece of territory and, ultimately, to project power out from this territory.