This chapter analyses the effects of the Partito Comunista Italiano's (PCI) negative engagement with the mass movements for the party itself as well as the movements. The history of the 1972–7 cycle of contention demonstrates that processes of innovation, diffusion and engagement can be driven by illegal and violent tactics, as well as by protest tactics which are no more than rowdy. It also shows that the engagement phase is a contingent political process, whose outcome depends on the actions of the gatekeeper rather than on the nature of the innovations of the cycle. This chapter also discusses the lessons that can be learned about the interaction between social movements and the ‘gatekeeper’ parties.