The major sources for the life and rule of Marcus are patchy and frequently unreliable. The biographies contained in the Historia Augusta claim to be written by a group of authors at the turn of the 4th century, but are in fact written by a single author (referred to here as "the biographer of the Historia Augusta") from the later 4th century (c. 395). The later biographies and the biographies of subordinate emperors and usurpers consist largely of lies and fiction, but the earlier biographies, derived primarily from now-lost earlier sources (Marius Maximus or Ignotus) are much more reliable. For Marcus Aurelius' life and rule, the biographies of Hadrian, Pius, Marcus and Lucius Verus are largely reliable, but those of Aelius Verus and Avidius Cassius are partly invented.
A body of correspondence between Marcus Aurelius' tutor Marcus Cornelius Fronto and various Antonine officials survives in a series of patchy manuscripts, covering the period from c. 138 to 166. Marcus' own Meditations offer a window on his inner life, but are largely undateable, and make few specific references to worldly affairs. The main narrative source for the period is Cassius Dio, a Greek senator from Bithynian Nicaea who wrote a history of Rome from its founding to 229 in eighty books. Dio is vital for the military history of the period, but his senatorial prejudices and strong opposition to imperial expansion obscure his perspective. Some other literary sources provide specific detail: the writings of the physician Galen on the habits of the Antonine elite, the orations of Aelius Aristides on the temper of the times, and the constitutions preserved in the Digest and Codex Justinianus on Marcus' legal work. Inscriptions and coin finds supplement the literary sources.