Why are some civic associations better than others at getting—and keeping—people involved in environmental activism? From 350.org to the Sierra Club, membership-based civic associations constantly seek to engage people in civic and political activity. What makes some more effective than others? A growing body of experimental research on voting demonstrates the importance of interpersonal interactions in mobilizing voters. In the modern political age, however, citizens participate in politics in myriad ways that go far beyond voting. What are the sources of that activism? This paper examines field experiments designed to test different strategies for getting people to participate in online and offline activism around environmental issues. These studies find that reinforcing peoples' identities as activists—by referencing their past words and actions or engaging with them as activists—has a powerful effect on their willingness to take environmental action. These studies provide evidence consistent with an emerging new conceptualization of activism as the product of dynamic social interactions.