The international movement of people represents a significant channel of knowledge diffusion, often complementing other transmission mechanisms. The definition of “international migrant” may vary in the different jurisdictions.1 Following the UN Recommendations,2 the concept implies a change of the country of residence and encompasses several categories of people: workers, persons who have the right of free establishment or movement deriving from international agreements, persons who are admitted for settlement, students, trainees, persons allowed to join for family reunification, persons admitted for humanitarian reasons (refugees, asylum seekers, etc.). Some other types of international mobility are not considered “migration” (for example, border workers or business travellers). A relevant difference relates to the intended duration of the stay in the receiving country, which may be temporary or on a long-term basis. Furthermore, migration flows may concern skilled or low-skilled people. Although all types of flows may entail knowledge-diffusion processes, in the following sections we consider in particular the international movement of workers (labour mobility) and of persons providing services, along with mobility taking place for on-the-job training, research purposes, education, etc. (see also Chapter 3 on regulations concerning the mobility of academics and students). International labour mobility, whether on a short-or long-term basis, enables highly-skilled and skilled people to transfer their knowledge to other countries, while less-skilled individuals acquire know-how and work experience abroad which eventually can be transferred to their home country (or to third countries). Persons moving for the provision of services (GATS Mode 4) can transfer valuable knowledge to the recipient country, as well as to their country of origin (OECD 2006b). For example, traded technologies often need to be complemented by know-how of highly skilled technicians, offering opportunities for learning and for technological improvements. Moreover the temporary movement of people to study abroad or to take on-the-job training enables individuals to use the acquired skills at home. In fact, several students from developing countries go back to their countries of origin, even if education abroad often represents a major avenue to find work in the host country.