What is the problem that solidarity is invoked as a solution to? How are solidarity schemes narrated? Which particular interests are pursued in its name? In this book, leading authorities in law, philosophy and political sciences respond to the solidarity question, drawing on debates on international law, international aid, collective security, joint action, market organization and neoliberalism, international human rights across the North/South divide, African mobility, transnational labour in the digital age and populism. This volume captures the shifting nature of long held historical assumptions on solidarity. Its twelve chapters open up for differentiated understandings of solidarity in law and politics beyond discursive cliché or ideological appropriation, bringing crises of the past into conversation with the crises of today. This book is also available as Open Access on Cambridge Core.