In this study, we use cross-country bilateral data to quantify a two-step process of international migration and its aggregate determinants. We first analyze which country-specific factors affect the probability that individuals join the pool of potential (aspiring) migrants. Then, we consider the bilateral and destination country factors that affect the frequency at which potential migrants turn into actual migrants. Using information on potential migrants from World Gallup surveys and on actual migrants from national censuses for 138 origin countries and 30 major destinations between 2000 and 2010, we analyze economic, policy, cultural, and network determinants of each step. We find that the size of the network of previous migrants and the average income per person at destination are crucial determinants of the size of the pool of potential migrants. Economic growth in the destination country, on the other hand, is the main economic generator of migration opportunities for a given pool of potential migrants. We also find that college-educated exhibit greater actual emigration rates mainly because of better chances in realizing their immigration potentials, rather than because of higher willingness to migrate. [ABSTRACT FROM AUTHOR]