The conditions under which Dutch trade unions have to operate have changed since the 1990s. Dutch trade unions have suffered a decline in membership and a loss of institutional power. They have been redefining their role in the socio-economic decision-making process and inventing new ways of representing the interests of the working population. The Dutch labour market has changed significantly over the last two decades, with employment becoming increasingly flexible. The number of migrant workers, mostly from Central and Eastern Europe, in the low-skill segments of the Dutch job market has increased, especially since the EU enlargement rounds in 2004 and 2007. These workers are embedded in a labour market governed by host and home-country regulations as well as EU legislation. In view of these developments, this chapter discusses how trade unions in the Netherlands have defined their position towards immigration and migrant workers, and whether and how they have included these workers in unionism.