The Roman Empire's administrative geography — the province system that evolved from the Republic through the Empire — is one of the most important topics in ancient history and one that receives surprisingly little popular attention. By the 4th century AD the empire was divided into roughly 100 provinces, organized into prefectures and dioceses under a hierarchical administrative structure that was the direct ancestor of medieval European administrative geography.

The creation of a province was a legal act: the senatus consultum that created a province established its boundaries, defined its legal status (regarding Roman law), and empowered a governor with specified authority. The governor combined military command, judicial authority, and fiscal oversight in a single magistracy that had no exact modern equivalent.

The evolution from Republican provincial governance (where senatorial governors competed for lucrative commands) to Imperial governance (where the emperor directly controlled the most important provinces) happened gradually across the reign of Augustus. The distinction between 'senatorial provinces' (governed by proconsuls drawn by lot) and 'imperial provinces' (governed by legates appointed by the emperor) persisted in formal terms until Diocletian's reorganization made the distinction obsolete.

Diocletian's reform — doubling the number of provinces, reducing their size, and separating military from civilian command — was a response to the security crises of the 3rd century. The proliferation of officials it required was one of the factors driving the expansion of the late Roman bureaucracy and the associated fiscal pressures.