Primeros Memoriales is published here for the first time in its entirety both in the original Nahuatl and in English translation. The volume follows the manuscript order reconstructed for the Primeros Memoriales by Francisco del Paso y Troncoso in his 1905-1907 facsimile edition of the collection of Sahaguntine manuscripts he called Codices Matritenses. During the 1960s, Thelma D. Sullivan, a Nahuatl scholar living in Mexico, began a paleographic transcription of the Primeros Memoriales, along with an English translation. After Sullivan's death in 1981, a group of her colleagues finished, enlarged, and annotated her project. This long-awaited publication makes available to specialists and interested laypersons alike an invaluable portion of the remarkable Sahaguntine treasure of information on sixteenth-century Aztec society.