In this chapter I introduce the interpersonal perspective on emotions. I define what I mean by emotion and how emotions are different from the related concepts of affect and mood. I provide a brief historical overview of theorizing and research on emotion, which clarifies that the effects of emotions have traditionally been conceptualized and studied at the individual level of analysis. I then discuss prior theorizing that has been influential in pushing towards a more social conception of emotions, including work by Charles Darwin, Paul Ekman, Nico Frijda, Tony Manstead, Dacher Keltner, Brian Parkinson, Agneta Fischer, and several others. I define the scope of the book by describing the essence of the interpersonal approach to emotion and explaining how this approach is different from other existing perspectives. The chapter concludes with an overview of the book. The first part of the book (Chapters 2-4) lays out emotions as social information (EASI) theory. The second part of the book (Chapters 5-9) provides a comprehensive review of research on the social effects of emotions. The third part of the book (Chapters 10-11) summarizes the current state of the art, highlights implications for theory and research, and discusses caveats and future directions.