This paper deals with Polybius' theory of anacyclosis. In the sixth book of his Histories1 Polybius discusses the various constitutions of the various political systems, because he considers the constitution to be the main determinant of whatever happens in the socio-political field, and hence the main determinant of Rome's rise to world power. In this connection Polybius develops a special theory of constitutional change. The summary representation given in 6.4.7--10 paints the following picture: on the one hand, there are three positively valued constitutional types (kingship, aristocracy and democracy) and, on the other, their degenerate forms (tyranny, oligarchy and ochlocracy). The starting point of the constitutional cycle of any political system is seen as being a state of nature that is already hierarchically structured. After kingship has evolved out of this state of nature, the three ‘good’, the three ‘bad’ constitutions and the one pre-state type of constitution succeed each other in a continuous, law-abiding cycle. Thus the order is monarchy, kingship, tyranny, aristocracy, oligarchy, democracy, ochlocracy, monarchy, and so on . . .