This chapter first looks at the primary sources available for the study of sexuality in Tenochtitlan in the sixteenth century, including paintings, sculptures, buildings, prayer books, legal codes, letters, chronicles, and judicial documents. Among the sources, the work directed by the Franciscan Fray Bernardino de Sahagún is the most prominent. The chapter then addresses Nahua principles of sexuality, which were linked to fertility, pleasure, and moderation in sexual activities. When the principle of moderation was not followed, the consequences could be fatal for the community. The differences between social classes in regard to how people should conduct their sexual lives are looked at next; then, the different sexual practices, paying particular attention to attitudes towards these in moral discourses and texts written shortly after the Spanish conquest. In particular, abundant information is given about adultery, prostitution, gender identity, and same-sex relations. Finally, discourses aimed at women exalted virginity before marriage and fidelity to one’s spouse afterwards. By contrast, discourses addressed to men acclaimed the early self-discipline that would be rewarded with a successful marriage and beautiful children.