With the Aztecs, archaeology merges fully with history. The picture and rebus writing of the Aztec manuscripts and the accounts of Bernal Diaz del Castillo and Bernardino Sahagun are sources as important as the monuments of Tenochtitlan which lie beneath the streets of modern Mexico City.

The Aztecs were among the last of the wild Chichimecs to enter the valley of Mexico after the fall of Tula. For a time the Aztecs were vassals to some of the more civilized city states of the valley, such as Colhuacan, and by the middle of the fourteenth century the Aztecs had taken up residence on small islands in the marshy lake of Texcoco. The Aztecs allied themselves with the Tepanecs, who rule Azcapotzalco, but later turned on their allies and subjugated them in 1428.

The great Moctezuma I, one of the Aztec earlier kings, then embarked on a policy of expansion and conquest beyond the valley of Mexico, and one of his successors, Ahuitzotl (1486 – 1502), welded together an empire which reached from coast to coast and from the valley of Mexico to Guatemala.

The Aztecs, however, were never completely supreme in Mesoamerica, as the Incas were in Peru. The Tarascans, some of the Mixtecs, and even the Tlaxacans, who were their near neighbours, held out against them, a circumstance that was to help the Spaniards bring about their downfall.

In one of the great moments of history, Cortez and his men finally beheld the Aztec city of Tenochtitlan after their march inland from Veracruz in 1519. Bernard Dial del Castillo describes this confrontation of two worlds in simple but vivid prose.

Within the lake of Texcoco and all around its edges were countless dwellings. Three causeway led from the mainland to the island center of the city , and a grid of canals laced the Metropolis. The lake and the canals were filled with canoes and the whole scene was alive with people. Estimates indicate that the full zone of the city embraced 60,000 dwellings and 300,000 persons.

At the heart of Tenochtitlan was the great Pyramid with its twin temples dedicated to Huitzilopochtli, the Aztec god of war, and the Tlaloc, the Aztec god of rain. Nearby were other lesser pyramids and temples, and in the plaza stood the infamous skull-rack with its grisly exhibit of the thousands of human sacrifices to the nation's deities. The whole of this central, sacred precinct was enclosed by a wall, and just beyond the wall were the palaces of the Aztec emperors, including that of the tragic Moctezuma II, who was to face the invaders bemused by the idea that the bearded Cortez was Topiltzin Quetzalcoatl returned from beyond the sea.

Along the street and canals of Tenochtitlan houses stood on stone-faced platforms. They resembled the dwellings of ancient Teotihuacan and Tula in that the rooms for cooking, sleeping, eating, and storage were arranged around a central courtyard. The walls were of stone and adobe and roofs were formed of wooden beams and poles. The great palaces tended to follow the same plan. Most buildings in Tenochtitlan were whitewashed and provided a glittering setting for the gaudily painted stucco-covered pyramids dotted the city. Each of 20 clans was said to have had its own plaza, temple, and market in its own part of the city; and these clan holdings were grouped, again, into four larger quarters, also with their own plaza ceremonial centers and markets.

Tenochtitlan stood on the least desirable portion of land (and water) in the Valley of Mexico, but the Aztecs may have chosen the site because it was the only location available and could be approached on foot only over the fortified causeways. The growth of the city of Tenochtitlan and the eventual pattern that it took also was conditioned by chinampa farming, the method of cultivation whereby artificial gardens or plots of land are built up of water vegetation and lake bottom muck. The system is still practiced today in the Xochimilco section of the valley of Mexico, so the technique is well known. Chinampa beds are prodigiously fertile. Their bumper yield, plus the relative case with which foodstuffs could be moved by canoe over considerable distance, formed the economic basis of the city. One of the observations which the Spanish made about Tenochtitlan was that the houses in the outlying sections of the lake, among the Chinampas, tended to be simpler than those in the central sections of the city. These lake suburbs were, apparently, the dwellings of the farmers whose produce sustained the urban populace.

Tenochtitlan was a hive of activity for both merchants and artisans. Not only were merchants and the markets a part of the immediate life of the city, but some of the activities of the merchant group extended beyond the city to distant portions of the Aztec empire and to other nations. These merchant, pochteca as they were called, traveled widely on trading expeditions on behalf of the state an also served as emissaries and spies. The artisans of the city – potters, jewelers, featherworkers, and metal-workers – were craft specialists. Luxury articles for the aristocracy accounted for much of what they produced.