Effective civil society activism in the high politics realm of international peace and security has not received sustained scholarly attention, and, at least until recently, was considered a ‘hard case,’ compared to other issue areas. This article reviews recent civil society efforts and assesses, in a preliminary fashion, some of the preconditions and constraints on transnational civil society activism in a range of security issues, from antipersonnel landmines to antinuclear campaigns. It concludes that high levels of policy uncertainty, the possibility of issue reframing, significant resources, and strategic partnerships are all key ingredients for effective civil society engagement. Conversely, vague or diffuse goals, the absence of state engagement, and policy stovepipes, all stand as obstacles to transnational activism.