There are at least two Hilarys of Poitiers: one who can construct clear historical narrative, utter an eloquent plea to the emperor Constantius and write good commentary on biblical texts; another given to clipped sentences with tortured logic and incomprehensible phrases. This latter Hilary expounded the doctrine of God, or at least aspects of the doctrine of God, in a treatise usually called, as here, De trinitate (though whatever Hilary's title was, it was not that). The work's genesis and composition are the subject of this monograph. Hilary finally ‘published’ it in the present form of twelve books, numbering and summarising them toward the end of book i; internal references and correspondences reveal the underlay of an earlier work or works. Carl Beckwith, building upon and attempting to refine the analysis proposed by Manlio Simonetti, envisages three stages: a book to which the title De fide has been given (=the present books ii and iii of De trinitate minus later additions) written to justify his doctrinal position and ‘published’ after the conciliar affair at Biterrae in 356 when Hilary was exiled; a book or the draft of a book (I am not clear what status Beckwith ascribes to it) called by Beckwith Adversus Arianos (though Hilary never names them ‘Arians’), having as a corpus vile the letter of Arius to Alexander of Alexandria (=De trinitate books iv–vi); the final form of the work, including the freshly composed De trinitate, books vii–xii, plus book i which was added as a preface in 358 when Hilary planned and completed all twelve books with unifying additions. Beckwith's analysis is plausible and, to my mind, an improvement upon Simonetti's two-stage. I am willing to believe in this Adversus Arianos. Beckwith responds to the text well and picks up verbal hints and correspondences with a sharp eye for detail that evokes trust. His is a ‘supplementary’ (to use the categories of pentateuchal analysis) rather than ‘documentary’ hypothesis; consequently not everything fits: the putative De fide cannot have baldly started, as he says it did, ‘Pater est ex quo omne quod est constitit’. The reader who wants to learn from Beckwith must bear with uncertainties but will surely profit from this analysis and from his words on Hilary's theological method and historical setting. He rightly says that books vii–xii show a marked improvement in depth. Indeed, I think that the presentation of Christ as ‘anthropophany’ besides ‘theophany’ in book xi is strikingly imaginative and probably original, for I know nothing like it. I regret that Beckwith says several times that Hilary wrote about the ‘Triune God’: the designation is anachronistic as is De trinitate for the title of a book which is principally about the Godhead of the Son. Hilary may have thought that the Holy Spirit is God absolutely but he did not say so.