This thesis develops and uses a new panel dataset with stocks and emigration rates to the 6 main OECD receiving countries, by skill level, from 172 sending countries, from 1975 to 2000. Thanks to this new dataset, we show that skilled migrations have increased during the 30 last years but this phenomenon happens in a context of general increase of mobility and global rise of educational levels. This dataset allows us to highlight that, contrary to the common knowledge about the traditional literature, important inequalities in the country of origin of migrants imply an increase in skilled migrations compare to unskilled migrations. Furthermore, the dataset allows us to show that a gain from emigration is possible in poorest countries of origin, under the condition that skill emigration rates are not too large. From this point of view, projection of brain drain rates at the 2050 horizon underlines that intensification in « selective » policy from the main European receiving countries can turn out to be highly harmful for sending countries.