Activists make important strategic decisions about how to build social movements, which are often linked to the individual biographies and political views of participants. This chapter focuses on those who supported the Tea Party, the activists who took part in collective action, and how the insurgency was organized. We begin by developing estimates of the number of Tea Party activists, concluding that between 140,000 and 310,000 citizens took part. Tea Party activists were atypically conservative, and self-identified as evangelical Christian compared to supporters and the general US population. The second section of the chapter analyzes the mobilizing structures created by the Tea Party, especially the national umbrella groups that emerged to sustain the insurgency, and the local chapters maintained by activists. The mobilizing structures adopted by the Tea Party greatly facilitated its rapid expansion, but individual groups were almost entirely independent. As a result, coordinated action became difficult to sustain over time. The thin mobilizing structures of the Tea Party we document in this chapter are crucial to understanding the insurgency’s rapid decline.