From the Occupy protests to the Black Lives Matter movement and school strikes for climate action, the twenty-first century has been rife with activism. Although very different from one another, each of these movements has created alliances across borders, with activists stressing that their concerns are not confined to individual nation states. In this book, Daniel Laqua shows that global efforts of this kind are not a recent phenomenon, and that as long as there have been borders, activists have sought to cross them. Activism Across Borders since 1870 explores how individuals, groups and organisations have fostered bonds in their quest for political and social change, and considers the impact of national and ideological boundaries on their efforts. Focusing on Europe but with a global outlook, the book acknowledges the importance of imperial and postcolonial settings for groups and individuals that expressed far-reaching ambitions. From feminism and socialism to anti-war campaigns and green politics, this book approaches transnational activism with an emphasis on four features: connectedness, ambivalence, transience and marginality. In doing so, it demonstrates the intertwined nature of different movements, problematizes transnational action, discusses the temporary nature of some alliances, and shows how transnationalism has been used by those marginalized at the national level. With a broad chronological perspective and thematic chapters, it provides historical context, clarifies terms and concepts, and offers an alternative history of modern Europe through the lens of activists, movements and campaigns.