This monograph undertakes a comprehensive study of the formulaic language of the principal Spanish epic poem, the Cantar de mio Cid. First and foremost, the author analyzes the relationship between orality and the formulas, as well as the problems that this fact presents engendered by this relationship. Then, the numerous definitions of the formula and formulaic expression that have been proposed are reviewed, stressing the improvements that each of them strives to bring. Likewise, the book theorizes the notions of variation and fixation of these sequences, as well as the types of formulas, their function, their effects, the potential distinction between the expressions of the first hemistich and those of the second, and the quantitative data. All this furthers the working of the formulaic system, which acts on three levels: composition, textual constitution, and reception. The second part of this monograph contains a repertory of formulas and formulaic expressions, a task that until now has not been undertaken in its entirety. This book constitutes the most thorough study of the formulas of the Cantar de mio Cid, because it offers a theorization of the formulaic system and explores closely the modus componendi of the anonymous Castilian poet. As a result, this monograph is essential to better understand this poem better, and it is also useful because of the way in which the Cid relates with the other epic compositions and medieval texts that use this technique. The use of fixed and variable expressions, one of the ways of repetition, is very common in the medieval poetic genres. Among them the epic is undoubtedly the one that uses these sequences the most, and these are the basis for the composition of the works, giving them a specific esthetic.