As an emperor, Trajan's reputation has endured – he is one of the few rulers whose reputation has survived nineteen centuries. Every new emperor after him was honoured by the Senate with the wish felicior Augusto, melior Traiano (that he be "luckier than Augustus and better than Trajan"). Among medieval Christian theologians, Trajan was considered a virtuous pagan. In the Renaissance, Machiavelli, speaking on the advantages of adoptive succession over heredity, mentioned the five successive good emperors "from Nerva to Marcus" – a trope out of which the 18th-century historian Edward Gibbon popularized the notion of the Five Good Emperors, of whom Trajan was the second.
As far as ancient literary sources are concerned, an extant continuous account of Trajan's reign does not exist. An account of the Dacian Wars, the Commentarii de bellis Dacicis, written by Trajan himself or a ghostwriter and modelled after Caesar's , is lost with the exception of one sentence. Only fragments remain of the Getiká, a book by Trajan's personal physician Titos Statilios Kriton. The Parthiká, a 17-volume account of the Parthian Wars written by Arrian, has met a similar fate. Book 68 in Cassius Dio's Roman History, which survives mostly as Byzantine abridgments and epitomes, is the main source for the political history of Trajan's rule. Besides this, Pliny the Younger's Panegyricus and Dio of Prusa's orations are the best surviving contemporary sources. Both are adulatory perorations, typical of the late Roman era, that describe an idealized monarch and an equally idealized view of Trajan's rule, and concern themselves more with ideology than with actual fact. The tenth volume of Pliny's letters contains his correspondence with Trajan, which deals with various aspects of imperial Roman government, but this correspondence is neither intimate nor candid: it is an exchange of official mail, in which Pliny's stance borders on the servile. It is certain that much of the text of the letters that appear in this collection over Trajan's signature was written and/or edited by Trajan's Imperial secretary, his ab epistulis. Therefore, discussion of Trajan and his rule in modern historiography cannot avoid speculation, as well as recourse to non-literary sources such as archaeology and epigraphy.