Abstract: Cooperation can be sustained by institutions that punish free-riders. Such institutions, however, tend to be subverted by corruption if they are not closely watched. Monitoring can uphold a regime of honesty and cooperation, but usually comes at a price. The temptation to skip monitoring regularly leads to outbreaks of corruption and the breakdown of cooperation. We model the corresponding cycle by means of evolutionary game theory, using analytical methods and numerical simulations. The results confirm the view that transparency is a major factor in fighting corruption.