Since the late 1990s, the EU has sought to develop the so-called ‘external dimension’ of cooperation on immigration and asylum: attempts to manage migration through cooperation with migration sending or transit countries. However, one can discern two rather distinct concepts of the ‘external dimension’. The first involves attempts to externalize traditional tools of domestic or EU migration control; the second to prevent the causes of migration and refugee flows, through development assistance and foreign policy tools. Both are based on different assumptions about how best to influence migration flows, and will have divergent impacts on migration flows, refugee protection and relations with third countries. It therefore makes a big difference which of the two is likely to emerge as the predominant pattern of cooperation in the future. This article looks at the factors influencing the emergence of both concepts, focusing on three central determinants: the potential of such approaches to meet migration policy goals; the institutional context; and domestic political—electoral pressures. It argues that the two last factors have militated in favour of the prevalence of externalization approaches to the detriment of longer-term strategies of migration management, refugee protection and relations with third countries.