The book offers an analysis of the relationship between trade unions, immigration and migrant workers across 11 European countries in the period between 1990 and 2015. This introductory chapter explains the editors’ approach to this study, which is based on the comparative framework as developed in an earlier book by Penninx and Roosblad in 2000. This framework is critically reconsidered and its validity is checked in the light of recent contextual changes. It informs the development of the main questions that will underpin both the structure and content of the 11 country cases and the comparative analysis presented in the concluding chapter. In addition, this introduction addresses relevant methodological aspects and outlines the structure of the book.