This article analyses why British and Indian scientists working in Boston's pharmaceutical and biotechnology sector moved to the United States. Based on more than 200 structured interviews, the author examines two aspects of their migration. First, whether both groups differed in their reasons for migration and, second, how important economic and social network factors were in influencing their migration. He shows that there were distinct differences between both groups. British scientists were driven mainly by professional opportunities, whereas Indian scientists were mainly driven by educational opportunities. Both groups used social contacts to varying degrees when making the final decision to move to the United States.Keywords highly skilled migrants, social networks, Boston, pharmaceutical and biotechnology sector, human capitalThere is now an extensive literature on highly skilled migrants (Beaverstock,