This introductory chapter explains the aim of the volume and unpacks the shared assumptions and starting points before outlining the structure and content of the book. The chapter provides an overview of the polarity literature, and how it has evolved since the early Cold War. It summarizes the findings of the book and discusses their implications. In particular, the chapter highlights two conclusions from the book: 1) Polarity effects are weaker today than they were for most of the nineteenth and twentieth centuries, and consequently international politics are now more regional and less systemic than in the past century; 2) The United States and China stand out as the strongest powers, but regional powers and small states seek to navigate US-China rivalry from their own perspective rather than getting co-opted by one or the other.