As the introduction to the book, this chapter describes the overall motivation for writing the book, as well as its structure (the plan of the book). It offers a preliminary description of the phenomenon ‘coalition of the willing’, based on Eyal Benvenisti’s notion of ‘informal coordination mechanisms among like-minded States’. It connects the subject of this book within the existence literature and the trends identified so far by it, such as transnational networks, the move from institutions, informal international law-making, and global administrative law. At the same time, it makes clear in what sense the present book differs from those approaches, in particular as it traces the coalition of the willing approach to US foreign policy strategy. This chapter also makes clear why coalitions of the willing are relevant for international law, as they challenge fundamental categories like sovereign equality and State consent. It then moves on to explain how coalitions of the willing and the problems they pose to international law will be analyzed in the course of the book.