- Ruled by a king who had absolute power
- Senate of elders advised the king
- Comitia Curiata, an assembly of patricians, also had some power
Republic (509 BC-27 BC)
- Ruled by two consuls, elected by the Senate
- Senate had most of the power
- Comitia Centuriata, an assembly of all citizens, also had some power
Empire (27 BC-476 AD)
- Ruled by an emperor who had absolute power
- Senate and Comitia Centuriata lost their power
- Empire was divided into provinces, each ruled by a governor appointed by the emperor