The enlargement of the EU in 2004 and 2007 has led to greatly increased free movement of workers from 'new' to 'old' member states. The unprecedented scale of this migration has had a profound impact on the regulation of labour law in Europe. This book compares the ways trade unions have responded to the effects of the enlargements, and in particular to the increased migration of workers across borders. It undertakes a contextualised comparison of trade union responses in Austria, Germany, Ireland, Sweden and the UK, and examines the relationship between trade unions and labour law at a national and European level. This analysis illustrates how trade unions can use law to better respond to changing regulatory and opportunity structures, and indicates the kinds of laws that would benefit trade unions at a national and European level.