Some of the most consequential associations in American history were created with the aim of achieving fundamental change in our society. While the moral and social views of their membership varied, their sense of mission became a powerful engine for organizing members across the nation. Through time, Americans have created large, intricate networks of organizations dedicated to such social reforms as establishing racial and economic equity, addressing crises of morality and education, and meeting challenges to civil or religious liberties. Others fervently fought or sought to reverse change, regarding it as an unwelcome challenge to a social order based on exclusion. Whether making incremental or sweeping changes, impassioned Americans standing together have continually rewoven parts of the nation’s social fabric.