This introductory chapter elaborates the conceptual foundations of horizontal intergovernmental coordination and outlines the analytic framework for the following chapters. To begin with, the practical relevance of horizontal intergovernmental coordination is emphasised. In the second section, the building blocks of the concept are introduced in detail—a conceptualization of coordination, the relevant actors in intergovernmental relations, and the horizontal dimension of coordination. An argument for the necessity and benefits of a comparative perspective is developed in section “The Comparative Perspective”. In section “Structure of the Book”, the structure of the book is explained in detail. Specifically, the chapter templates for both the country reports and the case studies are described and explicated. The introduction closes with an outlook to the concluding chapter of the book, where insights from a comparative reading of both the country reports and the case studies are used to form hypotheses on patterns and drivers of coordination. Those hypotheses provide a starting point for further empirical investigation and theory building.