This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book is about punk and post-punk as social worlds or 'music worlds'. The transformation of punk into various forms of post-punk in Manchester, Liverpool and Sheffield between 1977 and 1980 focuses primarily upon the structure and dynamics of emergent music worlds. The book discusses the concepts of 'critical mass' and 'social networks'. It charts and explains the evolution of London's proto-punk network between January 1975 and December 1976, before punk 'went national'. The book reviews the most often cited explanations of punk in both the sociological and the wider literature, identifying strengths, flaws and gaps in these explanations. It considers how talk about punk migrated from face-to-face networks to mass media networks.