This paper is part of the joint project between the Directorate General for Migration and Home Affairs of the European Commission and the OECD's Directorate for Employment, Labour and Social Affairs on "Review of Labour Migration Policy in Europe". This document has been produced with the financial assistance of the European Union. The views expressed herein can in no way be taken to reflect the official opinion of the European Union. Grant: HOME/2013/EIFX/CA/002 / 30-CE-0615920/00-38 (DI130895) A previous version of this paper (DELSA/ELSA/MI(2015)10) was presented and discussed at the OECD working party on migration in June 2015 This paper examines the impact of the free flow of migrants within the EU on the prospects of labour migrants from third countries - the extent to which free movement migrants and third country migrants are substitutes or complements on the labour market. The first section of this paper looks at the recent trends in migration to the European Union, with a particular focus on trends in the 'big five' recipient countries. The analysis is supplemented by the use of micro data from the EU Labour Force Survey, to examine the extent to which the socio-economic and job characteristics suggest that EU migrants and third country migrants provide a similar labour input. Aggregate migrant flows, however, are driven by both supply and demand factors; a comparison of aggregate trends is therefore insufficient to disentangle the disparate drivers of these trends. A booming economy, for example, will attract labour migrants from both EU and third countries, yet the positive relation between these flows cannot be attributed to a complementarity between these labour inputs but rather to the demand side factors that drive them both. To overcome this endogeneity the second section of this paper utilizes the natural experiment of EU enlargement to isolate the impact of the increased supply of free movement migrants on third country migrant populations. Abstracting in this manner from the economic factors that have played such an important role in determining labour demand in recent years the empirical analysis of this paper identifies a negative impact on the arrivals of third country migrants when labour supply from new EU migrants increases. Furthermore, the lack of identifiable impact on the employment rate of third country migrants is dependent on assumptions regarding the counterfactual employment outcomes of these displaced third country migrants.