The Kyklos is a term used by some classical Greek authors to describe what they saw as the political cycle of governments in a society. It was roughly based on the history of Greek city-states in the same period. The concept of "The Kyklos" is first elaborated in Plato's Republic, chapters VIII and IX. Polybius calls it the "anakyklosis". (1)

According to Polybius, who has the most fully developed view of the cycle, it rotates through the three basic forms of government, democracy, aristocracy, and monarchy and the three degenerate forms of each of these governments ochlocracy, oligarchy, and tyranny. Originally society is in anarchy but the strongest figure emerges and sets up a monarchy. The monarch's descendents, who because of their family's power lack virtue, become despots and the monarchy degenerates into a tyranny. Because the excesses of the ruler the tyranny is overthrown by the leading citizens of the state who set up an aristocracy. They too quickly forget about virtue and the state becomes an oligarchy. These oligarchs are overthrown by the people who set up a democracy. Democracy soon becomes corrupt and degenerates into mob rule, beginning the cycle anew.

Plato and Aristotle have somewhat different view. Plato only sees five forms of government. Aristotle believes the cycle begins with monarchy and ends in anarchy, but that it does not start anew. He also refers to democracy as the degenerate form of rule by the many and calls the virtuous form politeia, which is often translated as constitutional democracy.

All the philosophers felt this cycling was harmful. The transitions would often be accompanied by violence and turmoil and a good part of the cycle would be spent with the degenerate forms of government. Aristotle gave a number of options as to how the cycle could be halted or slowed:

• Even the most minor changes to basic laws and constitutions must be opposed because over time the small changes will add up to a complete transformation.
• In aristocracies and democracies the tenure of rulers must be kept very short to prevent them from becoming despots
• External threats, real or imagined, preserve internal peace
• The three government basic systems can be blended into one, taking the best elements of each
• If anyone individual gains to much power, be it political, monetary, or military he should be banished from the polis
• Judges and magistrates must never accept money to make decisions
• The middle class must be large
• Most important to Aristotle in preserving a constitution is education if all the citizens are aware of law, history, and the constitution they will endeavour to maintain a good government.

Polybius, by contrast, focuses on the idea of mixed government. The idea that the ideal government is one that blends elements of monarchy, aristocracy, and democracy. Aristotle mentions this notion but pays little attention to it. To Polybius it is the most important and he saw the Roman Republic as the embodiment of this mixed constitution and that this explained its stability.