This paper focuses on the issue of scientific mobility within the European Research Area (ERA) and its impact on both the regions concerned and the individual scientists. It identifies a fundamental tension implicit within the ERA strategy between regional equality or ‘balanced growth' on the one hand, and individual equity on the other. Drawing on the findings of a series of recent research projects,1 and with a particular focus on an impact assessment of the European Commission's flagship scientific mobility scheme (the Marie Curie Fellowship Scheme), it considers the nature of scientific mobility in the EU context and the quality of flows. These findings are set within the wider literature around highly skilled migration (HSM) and brain drain/circulation which identifies a number of key variables shaping the impact of mobility: namely the direction and temporal characteristics of flows and their ‘quality' in terms of who is moving. It also considers the relationship between HSM and transfers of knowledge. Although knowledge is embedded in individual scientists—and, to that extent, moves with them—the process of knowledge transfer takes place in a variety of ways reflecting not only scientists' employment location but also the quality and extent of their networks and connections. The paper cautions against simplistic interpretations of statistics based on the volume and geography of migration flows. Whilst such findings raise important questions they are not constitutive of ‘brain drain' as such. The processes and impacts of HSM defy such simple conclusions and demand a more detailed analysis of the factors shaping migration (and return) and the relationship between these and transfers of knowledge. The paper also raises critical questions about the balance between European interests (and the economic ‘trickle down' that derives from these to Member States) and the specific concerns in terms of the science base of individual Member States.