This short monograph examines the authorship, date, context, redaction and reception of the Historia Augusta - a corpus of biographies of emperors and usurpers of the second and third centuries, which purports to be the work of six writers active in the reigns of Diocletian and Constantine. Thomson accepts the widely held view that one author, a scholarly impostor, composed and redacted the Historia Augusta some time after about 395. Internal evidence -which includes administrative anachronisms and allusions to events, as well as spurious names, genealogies and documents- suggests that the corpus was intended for an audience among the Roman elite of the end of the fourth century. Thomson argues that the lives were not written for a polemical purpose. Their author instead responded to widespread interest in the works of Suetonius and Marius Maximus; his countless fabrications represented attempts to fill lacunae in the record with material appropriate to the genre of imperial biography. To this end, the scholarly impostor plundered the tradition for literary models and historical examples, apparently unmoved by the strict demands of chronology.This monograph advances several arguments that may be considered innovative. After examining the evidence of the text and the tradition, Thomson substantively revises existing theories on the redaction of the corpus. He proposes that an extant collection of panegyrics (the Panegyrici Latini) -or some similar work now lost- may have provided a model for the otherwise baffling imposture of collective authorship and tetrarchic date. Thomson also tentatively suggests a connection between the scholarly impostor, the spurious author Flavius Vopiscus Syracusius and a Syracusan poetaster and antiquarian active in the relevant period (Naucellius).