Beginning with Thucydides’ history of the Greek settlements in Sicily, this thesis focuses specifically on his account of the foundations of Gela and Himera and their development during the Archaic period. Identified by Thucydides as being multi-ethnic yet having the institutions of a specific ethnic group, the relationship between ethnicity and institutions at these two cities is analyzed to better understand its role in identity formation. Focusing on the material and literary evidence from these two cities, as well as Thucydides’ account of their foundation, ethnicity at these cities is shown to be a highly mutable and varied form of identity that was constructed in reaction to contemporary political, social, and cultural needs, and co-existed alongside other forms of identity. These conclusions are placed in context by an overview on Greek ethnicity, colonization, institutions, as well as on material and literary indicators of ethnicity.