This chapter outlines a theory of micro-mobilisation, which explains the emergence of punk in London between late 1975 and the end of 1976. It argues that music worlds emerge as an effect of collective effervescence within a networked critical mass of actors who are defined by shared interests of some sort. The chapter also outlines a theory of the emergence of music worlds. Music worlds are a form of collective action and they arise through a process of mobilisation and collective effervescence. This is only possible, however, where a critical mass of interested individuals are connected to one another in a social network, or at least where sufficient interest among appropriately resourced individuals within a network can be mustered. The chapter considers the role of homophily and 'social space' in the process of world formation.