In fall 2009, an interdisciplinary team of roughly twenty scholars from six different countries set out to collect data on street demonstrations. At the time of this writing, almost 70 demonstrations are covered and nearly 15,000 demonstrators surveyed, and we expect to cover an additional ten demonstrations in the future. Since 2009 the team has grown. The project now has more than thirty participants from nine different countries. Involvement has grown from a focus on the six original countries (Belgium, the Netherlands, the UK, Spain, Sweden, and Switzerland) to now include scholarship on Italy, Mexico, and the Czech Republic. This special issue is a first report on the findings generated by what we believe is the largest comparative study ever of participants in collective action. Street demonstrations have become more and more common throughout the world. Almost daily, newspapers report on street demonstrations taking place in some city, somewhere. The research project, entitled Caught in the Act of Protest: Contextualizing Contestation (CCC), aims to increase our understanding of the dynamics of street demonstrations. Politics and societies have changed substantially during the last few decades (van Stekelenburg, Roggeband, and Klandermans forthcoming 2013). Increasingly, supranational political institutions have gained prominence and their impact on people’s daily lives has grown. At the same time, in many societies a new social fabric seems to be evolving. Loosely coupled networks have become a prime mode of structuring society, accelerated by the Internet, social media, and cell phones. In this new political and societal context, it remains poorly understood how people mobilize for change, who takes to the streets, and why. Studies of protest behavior tend to focus on single protest events or alternatively to employ general population surveys. Either type of study inevitably strips the data of contextual variation. Consequently, fundamental questions about how context influences contestation remain unanswered. Questions such as who participates in protests, why they participate, and how they are mobilized all lack, to date, comparative, evidence-based answers. The composition of the demonstrating crowd, the motivation of the participants, and the mobilization techniques that brought them to the streets are contingent on contextual variation, but, void of systematic comparison, we can only guess what the influence of the context may be. Tilly (2008) has argued that, like most contentious performances, street demonstrations obey the rules of strong repertoires. That is to say, participants in street demonstrations enact available scripts within which they innovate, but mostly in small ways. As a consequence, street demonstrations are the same and different every time they occur. Street demonstrations vary on a continuum from ritual parades to violent protest events. In this issue we will compare May Day parades and climate change demonstrations. Although May Day parades have a highly ritualistic character, they do vary in the political undertone they convey. climate change demonstrations, on the other hand, oscillate between ritual manifestations and more contentious events.