Why do petitions flourish when they are often denied if not ignored by the sovereigns who receive them? When activists seek to build political organizations in network-rich but information-poor environments, petitioning as institutional technology facilitates recruitment. A petition’s signatory list identifies and locates individuals sympathetic to its prayer and expresses to other citizens who and how many agree with the prayer. Three historical moments—the explosion of antislavery petitioning in the antebellum United States, the emergence of Protestantism in sixteenth-century France, and England’s suppression of petitioning after the Restoration Settlement of 1660—provide vivid demonstrations of the theory. A recruitment-based theory implies that petition drives mobilize as much as they express, that well-established groups and parties petition less frequently, and that the most important readers of a petition are those asked to sign it. The petition’s recruitment function complements, but also transforms, its function of messaging the sovereign. Contemporary digital petitioning both routinizes and takes its force from the petition’s embedded recruitment technology.