"During a happy period of more than fourscore years, the public administration was conducted by the virtue and abilities of Nerva, Trajan, Hadrian, and the two Antonines. It is the design of this and of the two succeeding chapters to describe the prosperous condition of their empire, and afterwards, from the death of Marcus Antoninus, to deduce the most important circumstances of its decline and fall, a revolution which will ever be remembered and is still felt by the nations of the earth."

So Edward Gibbon concluded the first paragraph of his massive The Decline and Fall of the Roman Empire, referring to a period which he also styled the happiest of mankind's history. Hadrian was the central figure of these "five good emperors," the one most responsible for changing the character and nature of the empire. He was also one of the most remarkable and talented individuals Rome ever produced.

The sources for a study of Hadrian are varied. There is no major historian for his reign, such as Tacitus or Livy. The chief literary sources are the biography in the Historia Augusta, the first surviving life in a series intended to continue Suetonius' Lives of the Caesars. Debate about this collection of imperial biographies has been heated and contentious for more than a century. The most convincing view is that which sees the whole as the work of a single author writing in the last years of the fourth century. The information offered ranges from the precisely accurate to the most wildly imaginative.

Cassius Dio, who wrote in the decade of the 230s, produced a long history of the empire which has survived, for the Hadrianic period, only in an abbreviated version. Fourth century historians, such as Aurelius Victor and Eutropius, occasionally furnish bits of information. Contemporaries or near-contemporaries of Hadrian, such as Arrian, Fronto, Pausanias, and Plutarch, are also useful. Papyri, inscriptions, coins, and legal writings are extremely important. Archaeology in all its aspects contributes mightily to any attempt to probe the character of a man and emperor whose personality and thoughts defy close analysis and understanding.