Thucydides’ History is a rich source for our understanding of the character and interrelations of the ethnic sub-groups of the Greeks and different communities within the Greek world, as well as the relations between Greeks and non-Greek (‘barbarian’) communities. After establishing some key methodological principles relating to studying ethnicity in the Greek world, this chapter explores Thucydides’ contribution to our understanding of Greek ethnicity. It analyses the role of descent and cultural factors in the construction of ethnicity. It also explores the role that ethnicity plays in Thucydides’ description and analysis of the Peloponnesian War.