Aztec astronomy is described in detail in the present chapter. Mention is made of the passages of the Sun at the zenith, of the observation of the equinoxes and solstices and of the observations of the Moon and planets. The Aztecs understood that they could observe the planet Venus as a “morning star” and as an “evening star” because they were two different aspects of the same planet. They also realized the commensurability of its synodic period and of the vague year. The information on Venus given in the Borgia codex is described in detail. The contribution of the Aztecs to the study of the sky is less than that of the Maya because this people did not make detailed observations of the sky and did not leave maps of it. Nevertheless some constellations were observed by the Aztecs (Pleiades, Belt of Orion, Hyades, Castor and Pollux, Southern Cross, \(\ldots \) ) which are discussed in the text. They did not attempt to predict celestial events like lunar and solar eclipses or understand the scientific reality of them. Some discussions of the chapter emphasize the presence of crossed sticks on some manuscripts interpreted as possible observatories, the orientation of selected buildings observed on certain archaeological sites, the major role played by astronomy in relation to the Venus-rain-corn complex and the fact that Aztec cosmology refers extensively to Huitzilopochtli, the Sun god.