Periodization is after archaeological layers at Uruk. Thus, Uruk XVIII–XIV are not part of the "Uruk period" proper but are comprised by the Ubaid period. The Uruk period proper corresponds to the layers Uruk XIV–IV, with the late phase Uruk IV lasting ca. 3300–3100 BC. Uruk III reaches up to 3000 BC and into the Early Dynastic period .
These early city-states had strong signs of government organization (though social stratification was not strongly evident until very late in this period and the beginning of the Early Dynastic Period, beginning around 3100 BC), evident even in items such as cheap, mass-produced beveled rim bowls which were made to be discarded. These bowls may have been handed out at community outings, such as large-scale constructions. The cities grew to cover up to 250 acres (1 km²) and support up to 10,000–20,000 people by the end of the period.