Offering a lively and lucid history of the idea-from Ancient Rome through the first European and American socialists and labor organizers, to twenty-first century social movements like Occupy Wall Street and Black Lives Matter-Hunt-Hendrix and Taylor trace the philosophical debates and political struggles that have shaped the modern world. Looking forward, they argue that a clear understanding of how solidarity is built and sustained, and an awareness of how it has been suppressed, is essential to warding off the many crises of our present: right-wing backlash, irreversible climate damage, widespread alienation, loneliness, and despair. Hunt-Hendrix and Taylor insist that solidarity is both a principle and a practice, one that must be cultivated and institutionalized, so that care for the common good becomes the central aim of politics and social life.