This volume offers the first critical edition of the vast Commentary on the Pentecostal iambic canon (traditionally ascribed to St John the Damascene) composed by Eustathius, archbishop of Thessalonica. The attribution of the hymn to the Damascene was, in principle, called into question by Eustathius himself, who eventually suggested to have it adopted into Damascene's paternity only out of ecclesiastical obedience. The Commentary is probably the last text Eustathius wrote. It can be regarded as the summa of his method of work, his style of exposition, his scholarly interests and literary tastes. Moreover, it can be read as the first Byzantine attempt to create a fusion between a method of work which originated from the exegesis of classical texts and the modes of theological interpretation connected in turn with liturgical experience and pastoral practice. The edition of the text is accompanied by three apparatuses, a complete range of indices, and exhaustive Prolegomena where the editors shed light on the Commentary as such - its genesis and date, its audience, its discussion of the traditional attribution, its sources - and on history of its manuscript tradition, with a special focus on the Constantinopolitan didaskaleion of Prodromos-Petra.