This dissertation traces communitarian reform in North America from the end of the War of 1812 to the Civil War. It is a social history of reform philosophy in which reform projects, the conflicts surrounding them, and efforts to move from theory to practice provide the context for studying ideas. Nineteenth-century communitarians challenged cultures of self-possession, exclusive ownership, competitive economies, and evangelical religion. These men and women also offered an alternative vision of freedom rooted in social selfhood and secular interdependence. In doing so, they faced increasing ideological disaffinity with other reformers, not by identifying different targets for reform but by articulating alternative philosophies of self and society that undergirded reform praxis. This was especially true in the evolving communitarian relationship to antislavery and abolition. The dissertation brings to focus the contradictions, tensions, and ambiguities among pre–Civil War reformers to understand the rise of capitalism and liberal self-ownership as a contingent, contested, and incomplete historical process. It further highlights a paradox whereby the dominant and revolutionary vision of freedom ascendant during the Civil War depended upon narrower conceptions of human nature and social life than alternatives that had been articulated before it.Better known to historians under the moniker “utopian socialism,” there is still no comprehensive history of pre–Civil War communitarianism, and the connections and interrelationships among disparate communal groups remain virtually unexplored. By studying communitarians as they moved from theory to practice across the Northeast and the Midwest, this dissertation adds a continental perspective to the history of early American and trans-Atlantic radical reform, which has to date focused almost exclusively on East Coast cities and the Atlantic basin. In analyzing the full careers of communitarians rather than the rapid rise and fall of communities alone, it shows a temporal longevity of sustained communitarian commitment despite the periodic demise of community building. Finally, the project is rooted in the first half of the nineteenth century but also provides an early modern setting for nineteenth-century communitarianism. It also suggests that, although diminished at mid-century, the pre-war communitarian tradition is a new context of origin for late nineteenth-century American Pragmatism. Both traditions emphasized a materialist conception of the mind, self-dispossession, and a commitment to social ethics rooted in empirical experimentation and the critical role of education in progressive social change.