The mobility of health professionals affects the performance of health systems, and increasingly so since the European Union (EU) enlargements in 2004 and 2007. This publication presents research on the gaps in knowledge about the numbers, trends, impacts and policy responses to this dynamic situation, in particular in Austria, Belgium, Estonia, Finland, France, Germany, Hungary, Italy, Lithuania, Poland, Romania, Serbia, Slovakia, Slovenia, Spain, Turkey and the United Kingdom.Conducted within the framework of the European Commission s Health PROMeTHEUS project, the research posed a set of questions of key interest to policy-makers:* What are the scale and characteristics of health professional mobility in the EU?* What have been the effects of EU enlargement?* What are the motivations of the mobile workforce?* What are the resulting impacts on health system performance?* What is the policy relevance of those impacts?* What are the policy options to address health professional mobility issues?In answering these questions, this publication offers policymakers evidence of the nature and extent of health professional mobility in the EU, analyzes its impact on country health systems and outlines some major policy strategies to address mobility.