Critics of social injustice argue that the agent of transformative social change will (or should) be a mass agent; namely, an agent that is large, complex, and geographically dispersed. Traditional theories of collective agency emphasize the presence of shared intentions and common knowledge, but mass agents are too large for such cohesion. To make sense of mass agency, I suggest a new approach. On the solidarity theory of mass agency, a mass agent is composed of (a) organizers who intend to (...) fight for social change and (b) supporters who are in solidarity with organizers. (shrink)