On Solidarity clarifies a key idea in struggles for a more just world. What does solidarity mean, and how can diverse movements build enough of it to change society? Organizer and political theorist Mie Inouye leads a forum on obstacles to collective actiontoday. Rejecting the language of “allyship” and the politics of deference, she makes the case for maintaining solidarity through conflict in durable institutions over time, none of which is possible without the hard work of good organizing. With responses from activists and theorists—from Astra Taylor and Sarah Schulman to Charisse Burden-Stelly, Jodi Dean, and Juliet Hooker—the forum helps us think about what solidarity is and what it requires. Also in this volume, Simon Torracinta shows how a universal basic income can pave the way toa more solidaristic society, Judith Levine considers how films have portrayed solidarity among women in the face of abortion restrictions, Gaiutra Bahadur suggests terrain for Black-Asian solidarities, and Mariame Kaba, Kelly Hayes, and Dan Berger offer key lessons from the world of organizing.