This special issue of Refugee Survey Quarterly presents the experiences of front-line staff in Médecins Sans Frontières (MSF) as they grapple with the implications of the global refugee crisis. Over the past 18 months, hundreds of thousands of people have moved from Africa and the Middle East to Europe, generating widespread media attention and considerable political wrangling. But for aid workers, this situation raises questions that get to the very heart of humanitarianism and its purpose in the contemporary world. How does an organisation like MSF, the pioneer of “ sans-frontierism ”, engage with the shifting politics of borders and migration? What, practically, does it mean to be a “borderless” organisation in a world where migration controls are such a big political issue? What are the implications of the refugee crisis for humanitarian principles and medical care? This introduction to the special issue brings some humanitarian dilemmas into focus, arguing that, in reaction to the migration crisis, aid agencies may have no option but to take a more robustly political approach.