The authorship and the dating of the Historia Augusta constitute one of the most important problems of classical philology - at least from the beginning of the 17^th c., when Isaac Casaubon gave this name to the collection of biographies of the emperors from Hadrian to Numerian.  The manuscript tradition records six authors (Aelius Spartianus, Iulius Capitolinus, Vulcacius Gallicanus, Aelius Lampridius, Trebellius Pollio, Flavius Vopiscus) and the biographies contain references to Diocletian or to Constantine, a fact that has long (at least from the beginning of the 1800s) led people to maintain that the work was composed at the end of the 3^rd or beginning of the 4^th c.  The collection, however, presents various problems.  First of all, there is a great confusion in the distribution of biographies: for example, only one of them is attributed to Gallicanus, against twenty lives ascribed to Iulius Capitolinus.  Moreover, they are woven together without order, and the same attributions to individual authors are at times in contradiction with the dates that can be inferred from reading them.  They also present various anachronisms: for example, Spartianus refers to Diocletian in three lives and to Constantine in one.  Notices are likewise lacking for the individual authors who might also be inferred from other sources.  Furthermore, the biographies contain many documents that are full of errors and contradictions, as one can deduce from comparison with other authors, which therefore appear most likely to be the result of forgery.  Lastly, the presence of errors and falsehoods tend to increase in the second part of the work.  All these things have led scholars, starting with Hermann Dessau in 1889, to consider them a forgery and a fairly late composition (for Dessau to be placed in the reign of Theodosius, for Otto Seeck in the reign age of Honorius).  The debate has raged since the end of the 1800s, with a great number of philologists taking part; these have fixed the period of composition at different times: the age of Julian the Apostate (Baynes, Hohl), the years from 392 to the middle of the 4^th c. (Hartke, Schwartz, Syme, Chastagnol, Mazzarino, Johne), and even up to the 5^th c. (Flach), while there have also been theories accepting - or at least not entirely rejecting - the testimony of the manuscript tradition.  Of fundamental importance for its influence and methodological rigor is A. Momigliano’s 1954 study, “An Unsolved Problem of Historical Forgery: The ShA,” in the Journal of the Warburg and Courtauld Institute 17: 22-46 (often reprinted and augmented with appendices).  This has clearly fixed the terms of the debate, claiming as a non liquet and demonstrating how all the theses advanced concerning the collection’s forgery and post-dating have been inconclusive.  On the other hand, a theory with less support - though followed by scholars of the caliber of Manni and Pasoli - is Mommsen’s 19^th c. argument claiming that the biographies were originally composed in the era of Diocletian and Constantine, but were then published in two successive editions, one around 330 AD, and another at the end of the 4^th c.  Today the positions are extremely varied and caution is required, given that new elements capable of changing the outlook are not forthcoming.  In any case, the thesis of forgery of the authors is now generally accepted.  [A. Balbo; tr. C. L. Caterine].