This dissertation traces this history of the idea of solidarity, focusing on its emergence as a political concept in the mid-1800s and following it through several 19th century social movements. Examining the writings of republican, Marxist, and anarchist thinkers, I indicate the way in which the term "solidarity" is used to signal a path between the twin threats of atomistic individualism and statist collectivism. In a period of tumultuous social upheavals, the overthrow of monarchies and the consolidation of capitalism and the nation-state, the invocation of the term expresses a desire for new kinds of collectivities, in which individual freedom and the common good are reconciled. By exploring past debates, I hope that this study can offer a useful intervention in contemporary political thought and practice by illuminating the relational and ethical basis of large-scale social and political change.