Although humanitarian crises, such as the ongoing mass exodus from Syria toward Europe, tend to focus global attention on migration, each year millions of people migrate to and from affected countries throughout the world. Progress has been made in understanding drivers of migration, and we have relatively good data on immigrant populations, but we lack information on how many people leave their country each year to settle elsewhere and who these emigrants are. The impact of migration on the individual and on sending and receiving communities and countries is only partly understood. Economic effects can be very different from the impacts on society and culture; some gain from migration, whereas others lose. The lack of knowledge creates systemic risks and uncertainties and frustrates public debate and the formation of effective policies. As high-level leaders convene to discuss such issues at the first United Nations World Humanitarian Summit, we outline priorities for migration data collection, research, and training.