Trade unions face multiple challenges at the beginning of the twenty-first century, including increased inward migration. The accession of eight countries from Central and Eastern Europe to the EU in 2004 in particular has created a new dynamic of labour migration in Europe, sometimes raising concerns about social dumping and a ‘race to the bottom’. In the context of the weakening of organised labour, the deregulation of national labour markets and the spread of rather precarious employment relationships, including irregular migrants, unions increasingly struggle to secure ‘equal pay for equal work’. Attempts to organise migrants are made further difficult not only by language barriers, but also by the fact that migrants are over-represented in those sectors of the economy where union support is traditionally weak. Thus, contemporary labour migration poses many challenges to trade unions, including intra- and extra-European migration, an increase in precarious forms of migrant labour and the task of organising migrants. This thesis seeks to examine how trade unions in four Western European countries, Austria, Germany, Ireland and the UK, respond to these challenges. With Germany and Austria on the one hand and the UK and Ireland on the other, two pairs of countries have been selected that are classified as coordinated market economies (CMEs) and liberal market economies (LMEs). Therefore, the thesis seeks to establish whether unions in CMEs respond differently to the challenge of contemporary labour migration than unions in LMEs, and if so, how possible differences can be accounted for. The main findings of the study suggest that there is considerable variation in union attitudes towards migrant labour. Broadly speaking, unions in LMEs like Britain and Ireland appear to be more open towards migrant labour than unions in CMEs like Germany and Austria. In particular labour market factors and the structure of collective bargaining in each ‘variety of capitalism’ appear to be of considerable importance in accounting for the variation in union attitudes towards migrants. However, while union policies are certainly influenced by such ‘structural’ factors, they are not wholly determined by them as unions have some agency in the way they frame issues such as immigration.