The latter days of the Roman Republic, which preceded the Roman Empire, were marred by political instability, as senators conspired against one another to hold onto power at the expense of the people. Political factions confiscated property of the common folk, abused the rule of law, and even killed one another in pursuit of their own goals — the most famous assassination being that of Julius Caesar in 44 B.C.

After Caesar’s adopted son, Octavian, defeated Mark Antony and Cleopatra in 31 B.C., he became ever more powerful — until the Roman Senate decreed him emperor with the title “Augustus” (“great” or “venerable”) in 27 B.C., in effect establishing the Roman Empire as we know it. (Over the next two centuries, Rome grew to become the most populous of the mega-cities of the ancient world).

For the next five centuries until its dissolution, Rome’s fortunes would oscillate wildly between prosperity and decline, as the fate of millions of people within its ever-expanding borders were intimately tied to the sentiments (and mental state) of its emperor. (Here are the 50 most powerful leaders of all time).

Not every emperor had the same influence on Rome’s people. While most Roman emperors ruled for many years, at one time there were different ones in a single year. Some emperors were good rulers and are remembered for their military victories and the edifices they built. Plenty of other ones were defined by their mediocrity and insignificance . 

Click here to see the longest reigning Roman emperors

24/7 Tempo has assembled a list of the 25 Roman emperors with the longest reigns by consulting the Britannica’s timeline and individual articles on every emperor in the unified or western Roman Empire. Details on their reign also came from Britannica. Note that the list includes only emperors who came to power between the birth of the empire in 27 B.C. and 476, the year the last of them, Romulus, was defeated by the barbarian soldier Odoacer. That event is considered to have marked the end of the Roman Empire in the west and of Ancient Rome.