Reference edition:

Scriptores Historiae Augustae, edidit E. Hohl, editio stereotypa correctior addenda et corrigenda adiecerunt Ch. Samberger et W. Seyfarth, vol. I, Leipzig 1971 (5. Auflage) (Bibliotheca Scriptorum Graecorum et Romanorum Teubneriana) 

This term is used to identify the biographies of the emperors from Hadrian to Numerian, i.e. from AD 117 to AD 285, with the exception of the period comprising AD 244-53.  It includes thirty biographies that refer to a much higher number of sovereigns or usurpers (as an example, the so-called “thirty tyrants”, thirty people who sought to take power between the fall of Valerian at the hands of the Sassanians in AD 260 and the ascension of Diocletian in AD 284, are treated in a single biography).  The manuscript tradition, essentially based on the codex Vaticanus Palatinus 899 of the 9^th c., attributes them to six different authors, but on the question of their attribution and dating see the author card.  Their importance is immense, since in many cases they constitute the only source for this extremely complicated period of Roman history.  The complete list of biographies is as follows, divided according to the authors attributed to the biographies by the manuscript tradition:

Vulcacius Gallicanus: Vita Auidii Casii

Aelius Spartianus: Vita Hadriani; Vita Severi; Vita Pescennii Nigri; Vita Antonini Caracallae; Vita Antonini Getae

Aelius Lampridius: Vita Commodi Antonini; Vita Diadumeni Antonini; Vita Antonini Heliogabali; Vita Alexandri Seueri

Iulius Capitolinus: Vita Antonini Pii; Vita Marci Antonini philosophi; Vita Veri; Vita Heluii Pertinacis; Vita Didii Iuliani; Vita Clodii Albini; Vita Opilii Macrini; Vita Maximini I; Vita Maximini II; Vita Gordiani I; Vita Gordiani II; Vita Gordiani III; Vita Maximi et Balbini.

Trebellius Pollio: Vita Valeriani I; Vita Valeriani II; Vita Gallieni; Vita Gallieni Salonini; Vita Triginta Tyrannorum; Vita Divi Claudii

Flavius Vopiscus: Vita Divi Aureliani; Vita Taciti; Vita Probi; Vita Firmi, Saturnini, Proculi, et Bonosi; Vita Cari, Carini, et Numerani

The biographies are of various lengths, which range from two chapters in the life of Valerian II to the sixty-eight in that of Alexander Severus, and make reference to the Suetonian model of treatment per species (Vita Maximi et Balbini 4: Sed priusquam de actibus eorum loquar, placet aliqua dici de moribus atque genere, non eo modo quo Iunius Cordus est persecutus omnia, sed illo quo Svetonius Tranquillus et Valerius Marcellinus). Very often, however, neither the criterion of ordering through rubrics nor that of chronology is followed; rather, one witnesses progressions that are at times entirely chaotic and that greatly favor the anecdotal.  From an ideological point of view, one can perceive a senatorial outlook that infuses all the biographies, in which words of praise are reserved for emperors like Probus, who sought to govern in harmony with Rome’s ancient institutions.  Scholars have further identified a characteristically pagan religious outlook, which comes to light especially through the criticism of Christian emperors.

In terms of stylistics, recent studies (Adams) have highlighted how no substantial differences in quality can be detected between the various biographies, even if in a few of them rhetoric assumes greater importance.

The debate concerning sources is very complex: these have been partially identified as a lost work of Marius Maximus, who lived between the 2^nd and 3^rd c. AD and was the author of twelve imperial biographies from Nerva to Heliogabalus, and the writings of Herodian, Dexippus, and Eunapius, in addition to lesser authors of whom only a few testimonies remain for us, such as Iunius Cordus and Valerius Marcellinus. Numerous similarities have been perceived between the Historia Augusta and the historians Sextus Aurelius Victor and Eutropius, but the relationship between them is not yet clear and is impossible to determine with precision whether the biographies of the Historia depend on the two historians or vice versa. [A. Balbo; tr. C. L. Caterine].