This thesis presents an edition and translation of Regula cantus firmi, a short (about 1,900 words) anonymous music theory text from the later Middle Ages, dedicated to the practice of plainchant. The specific information concerns the following topics: the Guidonian hand; the “musical” letters A–G; clefs; the low, high, and very high registers; the various hexachords, their properties, solmization, and mutation from one hexachord to another; and sixteen intervals from the semitone to the double octave. Considerable attention is given to the eight plainchant modes, with their finals and ranges; their classification as perfect, imperfect, pluperfect, mixed, and intermixed; the species of diapente and diatessaron of which they are composed; whether they employ B-natural, B-flat, or both; and how to distinguish plagal modes from authentic modes in ambiguous cases. Though the topics covered are those commonly found in medieval theory treatises, the value of Regula cantus firmi rests in its claim to present and describe those elements masters should teach “to all those who want to have instruction in the art of music”; thus it provides valuable information on the oral teaching of music theory during the Middle Ages. The edition and translation follow the principles of Mathiesen and Solomon, Greek and Latin Music Theory: A Style Guide for Text Criticism, Translation, and the Preparation of Camera-ready Typescript. The four manuscript sources used in this edition are D-Sl, Donaueschingen 250; I-Fl, Ashburnham 1119; I-FZc, 117; and I-Rv, C.105. Key words: Medieval music theory; oral teaching; solmization.