This study addresses the phenomenon of internationalization in higher education and is composed of three chapters. The following research question inspired the first chapter: What is the influence of characteristics (and activities) of researchers on the internationalization of the Higher Education Institutions in which they work? As the main results, I identify that the researcher's: (i) international academic experience, (ii) insertion into international collaborative networks, (iii) international co-authoring and (iv) experience in international publications, are four of the main factors at the individual level that are positively associated with the internationalization of Higher Education Institutions. The second chapter built from the Social Exchange Theory in order to address the following questions: What are the main rewards and costs that researchers perceive in their internationalization activities? How do these expectations of rewards and costs, perceived by researchers, affect their choices of international insertion? As the main results, I identify the following rewards expected by researchers: (i) professional opportunities generated from the internationalization activity, (ii) greater social approval (by the academic group of which they are part), (iii) greater autonomy in their teaching and research activities, and (iv) greater personal and professional security. On the other hand, high temporal, monetary, psychic and physical costs are among the factors that discourage researchers from seeking international insertion in their academic activity. The following question inspired the third chapter: What constitutes the internationalization of full time academic researchers ? I propose that the internationalization of the researcher can be conceptually defined as a phenomenon of four dimensions, called PRID: (i) place, (ii) relationship, (iii) impact and (iv) dissemination. The data obtained from in-depth interviews of 34 reasonably (or highly) internationalized researchers indicates that a researcher internationalizes his (her) academic activity when he (she) engages in academic activities outside his home country (place), and/or establishes relationships with academic actors outside his country (relationship), and/or achieves impact in terms of the wide use of his works (impact) and/or publishes his works in journals of international reach or in partnership with international authors (dissemination).