The sequence commentary emerged as a new branch of medieval commentary literature in the twelfth century. The sequence itself, sung in the Roman Mass, was a hugely influential genre - several thousands of sequence texts are known today - but the fact that the Middle Ages also produced commentaries on this liturgical poetry has been hitherto practically unknown and very few commentary texts have been edited. The present work is the first attempt at a broader presentation of the sequence commentary genre. It makes available in modern editions seven previously unedited expositions on the sequence Ad celebres rex for the feast of St Michael. Introductions to each edition discuss the motifs interpreted, the commentary technique used and the sources drawn upon. Manuscript interrelations and textual problems are also treated here. Editions of four prologues introducing collections of commentaries are also included. These texts, though not specifically tied to the commentaries on Ad celebres rex, are presented here since they provide useful evidence of the interpretative frameworks chosen by the commentators. The complex textual transmissions of these texts have required three different editorial methods, which are discussed in a separate chapter. A general introduction surveys the sequence commentary material found to date. From these textual witnesses - nearly a hundred manuscripts listed in an appendix - we may conclude that the genre flourished mainly in the fourteenth and fifteenth centuries. Most manuscripts present large collections of commentaries on sequences for the whole liturgical year, generally preceded by a prologue and sometimes accompanied by a corresponding group of hymn commentaries.