Sumer was one of the earliest civilizations located in Mesopotamia between the Tigris and Euphrates Rivers. The Sumerians developed advanced cities with specialized workers, complex institutions, record keeping, and early technology. They had a polytheistic religion and a social hierarchy ranging from priests and kings to enslaved peoples. The Sumerians made major contributions to writing, architecture, farming, transportation, and were among the first to establish empires in Mesopotamia.