The Amorites established cities on the Tigris and the Euphrates rivers and made Babylon, a town to the north, their capital. During the time of their sixth ruler, King Hammurabi (1792-1750 B.C.), Babylonian rule encompassed a huge area covering most of the Tigris-Euphrates river valley from Sumer and the Persian Gulf in the south to Assyria in the north. To rule over such a large area, Hammurabi devised an elaborate administrative structure. His greatest achievement, however, was the issuance of a law code designed "to cause justice to prevail in the country, to destroy the wicked and the evil, that the strong may not oppress the weak." The Code of Hammurabi, not the earliest to appear in the Near East but certainly the most complete, dealt with land tenure, rent, the position of women, marriage, divorce, inheritance, contracts, control of public order, administration of justice, wages, and labor conditions.

In Hammurabi's legal code, the civilizing trend begun at Sumer had evolved to a new level of complexity. The sophisticated legal principles contained in the code reflect a highly advanced civilization in which social interaction extended far beyond the confines of kinship. The large number of laws pertaining to commerce reflect a diversified economic base and an extensive trading network. In politics, Hammurabi's code is evidence of a more pronounced separation between religious and secular authority than had existed in ancient Sumer. In addition to Hammurabi's legal code, the Babylonians made other important contributions, notably to the science of astronomy, and they increased the flexibility of cuneiform by developing the pictogram script so that it stood for a syllable rather than an individual word.