The book is an in-depth study about why and how international organizations (IOs) govern in the field. Through a close look at the case of migration and asylum, the book shows that IOs govern global challenges not only through rules, standards, and numbers but also through practice on the ground. Drawing on ethnographic fieldwork at the European external border, interviews at the headquarters of seven organizations—the United Nations High Commissioner for Refugees (UNHCR), the International Organization for Migration (IOM), the European Border and Coast Guard Agency (Frontex), the European Asylum Support Office (EASO), and three humanitarian non-governmental organizations (NGOs)—as well as an extensive document analysis, the book demonstrates that field staff improvise to organize collective action on underregulated issues in the field. Headquarter staff consolidate and diffuse their improvised solutions across operations. Field and headquarter staff, who organize across organizational boundaries and levels, thereby exert considerable influence on the life choices of individual governance addressees and define how global issues are treated in practice. They directly regulate global issues in the spaces where they become virulent, in different locations across the globe. The book offers important findings on this provisional governance mode that operates at a low institutional threshold but largely determines the de facto governance of new, contested, and crisis-ridden global problems, which is crucial for understanding how IOs govern given the crisis of the multilateral order. It also offers important insights into the global governance of mixed migration.