The site that would become Teotihuacán had been occupied by small settlements since c. 900 BCE, with the major construction of the city commencing around 300 CE. Its peak was around 150 - 500 CE, by which time it was the sixth largest city in the world at the time with had an estimated population of 125,000 - 200,000, and was the dominant civilization of a vast area that included much of Mesoamerica.

Teotihuacán owed its growth more to trade than war. It was the centre for the Mesoamerican trade in obsidian and benefited from its proximity to major trade routes running from the Valley of Mexico to Veracruz and Monte Albán, developing trading links even as far afield as Kaminaljuyú in Guatemala and Tikal in the Central Maya region. This vibrant and cosmopolitan city dominated the valley of Mexico throughout the Classic Period and stands as the most significant cultural influence on later Mesoamerican civilizations, and yet, there is a mystery surrounding Teotihuacán, in that no definate conclusion of who its people were has been reached. No one knows what the Teotihuacános called themselves, or their city.

Teotihuacán began to decline around 500-650 CE. Speculation is that internal crisis, agricultural decline or the decay of the city's trade networks leading to economic collapse may have been the cause - possibly it was a combination of several factors. The city was destroyed c. 750, possibly burned by known invaders or an internal uprising, and its center abandoned.

Six centuries later, Teotihuacán's overgrown ruins impressed the Aztecs so greatly they used the pyramids for worship, to consult oracles and perform sacrifices. They variously described it as being built by the Toltecs, giants or gods, and mistook the residential palaces along the Avenue of the Dead for tombs. It was the Aztecs that gave the city the name we know it by: the "Place of the Gods".

The Teotihuacán Mapping Project has revealed that the city covered over 11 sq. miles ( 30 sq. km) and is laid out on a grid orientated consistently to 15 degrees 25' east of north, which aligns it with the sacred mountain Cerro Gordo and may also have astronomical significance. The main thoroughfare is the 4-mile long Avenue of the Dead, which is bisected at the central Ciudadela complex (or Citadel) by an east-west avenue, effectively dividing the city into quarters. All major buildings are orientated along this main axis. The Ciudadela complex itself is an enclosed area some 1300 ft (400m) square, and incorporates pyramids, two palaces, a large plaza and the Temple of Quetzalcoatl. Opposite the Ciudadela is another vast enclosed compound which was probably the city's main marketplace.

So in this bustling metropolis of some 200,000 people, how did people live? The Teotihuacán Mapping Project carried out by René Millon reveals that the principle residences in the city were one-storey apartment compounds, of which more than 2,200 have been identified and each of which housed an estimated 60-100 people. These dwellings had windowless exteriors for coolness and privacy, and rooms surrounding a spacious patio which often had a small altar in the centre. Many of the apartment compounds were decorated with magnificent murals, but the humbler artisan compounds were built of cheaper materials and undecorated. Teotihuacán being a cosmopolitan city, there were at least two wards specific to 'foreigners' - a Zapotec ward on the west side, and a Veracruz/Maya one on the east side.

Apartment neighbourhoods seemed to contain a mixture of classes and professions, but most of the elite lived in larger and more richly decorated homes mainly located not far from the two great boulevards of the Avenue of the Dead and the East-West Avenue. The Aztecs believed these low-lying buildings were the tombs of their ancestors, which gave the Avenue of the Dead its name. Residences of priests and nobles occupied ground near the pyramids away from the hustle and bustle of the streets. No 'royal palace' has been identified, although it has been suggested that the huge square enclosure of the Ciudadela complex might have served that role, and two apartment complexes have been discovered within that may have been the residences of royalty or high-level officialdom..