In this article, the authors focus attention on a poorly understood aspect of contentious politics: the interaction between the transnational diffusion of new forms of protest behavior and police practices in response to them. Studies of diffusion are usually limited to the diffusion of one kind of innovation by one set of actors to another, as in the diffusion of technical innovations from innovators to adopters. But collective action diffusion also produces a parallel and interactive sequence of “public order” reactions. Using the transnational countersummits that emerged around the turn of the century as their source of evidence, the authors focus on the coevolution of protester and police innovations across national boundaries. The authors’ major finding is that the mechanisms that cause protester and police innovations to diffuse are remarkably similar, even though they can combine in different ways at different moments: promotion, the proactive intervention by a sender actor aimed at deliberate diffusion of an innovation; assessment, the analysis of information on past events and their definition as successes or failures, which leads to adaption of the innovation to new sites and situations; and theorization, the location of technical innovations within broader normative and cognitive frameworks. The authors close with a speculative application of their findings to the recent diffusion of protester tactics and regime responses in the Middle East and North Africa.