The origin of the Aztec people is uncertain, but elements of their own tradition suggest that they were a tribe of hunters and gatherers on the northern Mexican plateau before their appearance in Mesoamerica in perhaps the 12th century; Aztlán, however, may be legendary. It is possible that their migration southward was part of a general movement of peoples that followed, or perhaps helped trigger, the collapse of the Toltec civilization. They settled on islands in Lake Texcoco and in AD 1325 founded Tenochtitlán, which remained their chief centre. The basis of the Aztec's success in creating a great state and ultimately an empire was their remarkable system of agriculture, which featured intensive cultivation of all available land, as well as elaborate systems of irrigation and reclamation of swampland. The high productivity gained by these methods made for a rich and populous state.

Under the ruler Itzcóatl (1428–40), Tenochtitlán formed alliances with the neighbouring states of Texcoco and Tlacopan and became the dominant power in central Mexico. Later, by commerce and conquest, Tenochtitlán came to rule an empire of 400 to 500 small states, comprising by 1519 some 5,000,000 to 6,000,000 people spread over 80,000 square miles (207,200 square km). At its height, Tenochtitlán itself covered more than 5 square miles (13 square km) and had upwards of 140,000 inhabitants, making it the most densely populated settlement ever achieved by a Mesoamerican civilization. The Aztec state was a despotism in which the military arm played a dominant role. Valour in war was, in fact, the surest path to advancement in Aztec society, which was caste- and class-divided but nonetheless vertically fluid. The priestly and bureaucratic classes were involved in the administration of the empire, while at the bottom of society were classes of serfs, indentured servants, and outright slaves.

Aztec religion was syncretistic, absorbing elements from many other Mesoamerican cultures. At base, it shared many of the cosmological beliefs of earlier peoples, notably the Maya, such as that the present Earth was the last in a series of creations and that it occupied a position between systems of 13 heavens and 9 underworlds. Prominent in the Aztec pantheon were Huitzilopochtli, god of war; Tonatiuh, god of the sun; Tlaloc, god of rain; and Quetzalcóatl, the Feathered Serpent, who was part deity and part culture hero. human sacrifice, particularly by offering a victim's heart to the sun god, was commonly practiced, as was bloodletting. Closely entwined with Aztec religion was the calendar, on which the elaborate round of rituals and ceremonies that occupied the priests was based. The Aztec calendar was the one common to much of Mesoamerica, and it comprised a solar year of 365 days and a sacred year of 260 days; the two yearly cycles running in parallel produced a larger cycle of 52 years.