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Is the brain drain a curse or a boon for developing countries? This paper reviews what is known to date about the magnitude of the brain drain from developing to developed countries, its determinants and the way it affects the well-being of those left behind. First, the author introduces alternative measures of the brain drain and characterizes its evolution over the last 25 years. Then, a review the theoretical and empirical literature is provided. Although the brain drain is a major source of concern for origin countries, it also induces positive effects through various channels such as remittances, return migration, diaspora externalities, quality of governance and increasing returns in education. Whilst many scientists and international institutions praise the unambiguous benefits of unskilled migration for developing countries, the analysis suggests that a small but positive rate of skilled emigration (between 5 and 10 percent) can also have favorable effects on development. Nevertheless, the current spatial distribution of the brain drain is such that many poor countries are well above this level, such as sub-Saharan African and Central American countries.