Kings: In the early days of Rome, the city-state was ruled by a king. The kings were elected by the Roman Senate and served for a term of one year.
Consuls: After the Roman Republic was established in 509 BCE, the kingship was abolished and replaced with a system of two consuls. The consuls were elected by the Roman people and served for a term of one year. The consuls were responsible for the day-to-day administration of the Roman state and for commanding the Roman armies.
Dictators: In times of emergency, the Roman Senate could appoint a dictator to serve for a term of six months. The dictator had absolute power and could override the decisions of the consuls.
Emperors: In 27 BCE, the Roman Republic came to an end and was replaced by the Roman Empire. The first Roman emperor was Augustus Caesar. The emperors were absolute rulers and held the titles of Imperator (commander) and Princeps (first citizen). The emperors were also the commanders of the Roman armies and the chief priests of the Roman religion.