“All in all, we know far too little about migration to be able to draw reliable conclusions. The main problem is the missing data,” Frans Willekens (Max Planck Institute for Demographic Research) says. For this reason, he and his colleagues Douglas Massey (Office of Population Research, Princeton University, USA), James Raymer (School of Demography, Australia National University, Canberra) and Cris Beauchemin (Institute National d’Études Démographiques, Paris, France) call on both the research community and on political institutions to take action.