The Aztecs represent a complex, class-based civilization, characterized by culturally diverse practices encompassed by the Nahuatl-speaking peoples of the Basin of Mexico region and its surrounding areas. The subject of much misinformation, the clash between Aztec peoples and Spaniards, provoked by the Spanish invasion, gave rise to an immense number of written sources. Native-authored, hybrid, and Spanish-authored texts all must be carefully considered, but the translation of a still-growing number of texts in the Nahuatl language has provided insights Spanish-language texts cannot. Other kinds of evidence about the Aztecs and how their ideas and identities survived also exist, including material remains and ethnographic evidence. While the word “Aztec” is used in several ways, I use “Mexica” in this book for the peoples of Tenochtitlan and Tlatelolco, either “Excan tlatoloyan” (Tribunal of three) or “Triple Alliance” for the expansive confederation of the late Postclassic period, and “Aztec” for the linguistically and culturally related peoples of the Basin of Mexico region to highlight the variety of ethnicities that constituted Aztec peoples. A brief early history from the time of the migrations into the Basin of Mexico to the founding of Tenochtitlan by the Mexica, guided by their deity Huitzilopochtli, also is covered.