In the advanced industrial democracies, migrant workers have become an integral part of the construction sector workforces. Migrant workers are a particularly appealing source of labor to construction firms hoping to cut costs and increase profits by relying on irregular forms of employment. As a result, construction trade unions are experiencing downward pressures on wages, deteriorations of labor standards, and declines in sectoral union density. While these trends are occurring across the advanced industrialized world, construction trade unions have not responded uniformly to these new challenges. This cross-national study seeks to uncover why unions behave differently towards migrant workers as they fight to maintain collectively bargained wage levels and working conditions. Based on evidence from Denmark, Germany, the Netherlands, Norway, Sweden, and the United States, this research project argues that union democracy and organizing capacity interactively explain why some construction trade unions behave more inclusively towards migrant workers than others.