A theory for the behaviour of cartels is advanced. The theory deviates from the received literature on three main points: Cartel behaviour is derived from literature on human behaviour rather than from principles of profit maximization, the theory focuses on commodity markets rather than being general, and it deals with the timing of formation, break down, expansion and dissolution. The theory is stated formally in a simulation model, which seems to replicate well the qualitative behaviour of cartels. Historical accounts of the timing of cartel events lend support to the theory.