This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book considers the very intersection of the two dimensions through which mood unfolds: temporality and spatiality. It addresses a long-standing problem in the conceptualisation of mood. In attempting to distinguish between mood and other affective phenomena, scholars have often cited the idea that feelings and emotions are intentional, that they are always in some way directed toward something, whereas mood is a fundamentally non-intentional state, which encompasses one’s total perspective without having intentional objects. The book discusses the methodology and results of a number of innovative studies conducted by the authors and their collaborators, in which they set out to investigate the contagiousness of positive and negative moods in adolescent friendship networks. It demonstrates that how the quartet’s plot becomes constitutive for the formation of a mood that makes itself ‘felt’ in the text.