The migratory phenomena most commonly studied are the mass movements, from peripheral countries to the centre, motivated by economic reasons, which give rise to quite distinctive migrant communities in the host country. However, in the past decade, growing attention has been paid to the migrations of qualified workers: business executives, health professionals, engineers, scientists. This type of migration has very different characteristics: it is marked by the diversity in the directions of fluxes, in the duration of migration, in the institutional modalities. Other kinds of causes are attributed to it: imbalance in the political and social relations in the world system, the social structure of the home countries, the strategies of multinational companies and organisations, incentive policies to qualified immigration, cooperation with developing countries.Accordingly, the integration of the migrants in the host societies is usually quite different: higher geographical dispersion, less contact with the home country communities, increased acceptation by the host society, inclusion in the formal work market, different dynamics in identity formation, cultural change and sense of belonging. The most commonly mentioned consequence of this phenomenon is the so called "brain drain", that is to say, that home countries suffer the loss of qualified personnel in whose training they have invested. But "brain gain" (the effects in the host country) and "brain exchange" (the benefits accrued by the circulation of skilled personnel between countries, the return of human resources after a training or work experience abroad, the creation of transnational networks) should also be considered.Scientists are part of this trend of the international mobility of the highly skilled, but they present some specific characteristics, that have also been studied by several authors. International organisations concerned with the definition of science and technology policies, such as the OECD and the EU have paid great attention to the issue of the mobility of human resources, fostering studies, developing indicators, creating working groups and issuing recommendations and directives to their member states. On a European level, the mobility of researchers is seen both as a problem, when it is the competition with the US that is at stake, and as something that should be supported, in the case of intra-European migrations (it is one of the axis of the European Research Area). However, inside Europe, blatant asymmetries can be found, between countries that attract large numbers of scientists (such as the UK) and countries that send out those same researchers (mostly from Eastern and Southern Europe). These asymmetries are mainly due to the structure of the national scientific systems: levels of funding, availability of resources, job offers in science, recruitment policies, flexibility or rigidity of the academic careers, opportunities for progression).