The Valley of Mexico is a highlands plateau in central Mexico roughly coterminous with the present-day Distrito Federal and the eastern half of the State of Mexico. Surrounded by mountains and volcanoes, the Valley of Mexico was a center for several pre-Columbian civilizations, including Teotihuacan, the Toltec, and the Aztec. The ancient Aztec term anahuac and the phrase "Basin of Mexico" are both used at times to refer to the Valley of Mexico.

The Valley of Mexico is roughly 35 miles (60 km) east-to-west and 50 miles (80 km) north-to-south. The valley is an endorheic basin, with no natural outflow. As a result, before the 20th century, much or most of the valley was covered with a chain of inter-related lakes, including Texcoco, Chalco, Xochimilco, Zumpango, and Xaltocan. These have since been drained and little of their original expanse remains. The greater Mexico City urban area covers nearly the entire valley even today.

Air pollution associated with this urban environment, consisting primarily of ozone, sulfur dioxide, and suspended particles, is trapped by the natural contours of the valley. In winter, thermal inversions contribute to the pollution problem. This has led many organizations, from the World Resources Institute to the Guinness Book of World Records, to designate Mexico City as the most polluted major city in the world. The pollution level has, however, decreased dramatically since the late 1980s and early 1990s. Whereas the air reached unhealthy levels for 355 days during 1991, it improved to 170 days in 2003 and 141 in 2004

The Valley of Mexico averages roughly 7350 feet (2240 meters) above mean sea level. Although the valley is located within a tropical zone, the high altitude keeps it relatively temperate. The annual median temperature is between 61° - 64° F (16° - 18° C) and seasonal changes are small. Night frosts can occur in the coldest months, but even in January the average high is 70° F. April and May are the warmest months. The rainy season, which usually lasts from late May until early October, accounts for 80% of the annual 33 inches (850 mm) of rainfall and moderates the usual dryness of the air.