The Aztecs were a Mesoamerican civilization that dominated much of present-day Mexico from the 14th to the 16th century. They had a complex society with a sophisticated system of government, religion, and writing, and a well-developed economy based on agriculture and trade. The Aztecs built great cities, including their capital, Tenochtitlan, which was located on an island in the middle of a lake and was one of the largest cities in the world at the time.