Issues related to group-based and -influenced behaviour have always been at the heart of social psychology. What is generally considered the first experimental study in social psychology - Norman Triplett's 1898 attempt to explain why bicycling speed records were faster when measured in competition against others - was a demonstration of group-influenced behaviour, and led to the development of the notion of social facilitation. The questions of how decision-making group members reach consensus, and whether they are a more effective vehicle for decisions than an insightful individual, remain popular 80 years after they were first introduced. Researchers in other disciplines are increasingly drawn to the group as a focus of study and of course, groups remain the standard decision-making unit for most important decisions in most societies. It is against this backdrop of the rich history, modern development and future potential of the subject that this timely four-volume set has been created, with the intention that it becomes a unique go-to resource for researchers in the field. The four volumes are arranged schematically: Volume One: Group Performance takes a look at individual action in the presence of others. Volume Two: Interdependence within Groups examines situations in which one is partially dependent on others for the magnitude of one's personal outcomes. Volume Three: Decision-Making Groups explores group settings in which there is full collaboration in order to produce a single output. Volume Four: Relations among Groups presents a set of articles on interaction between groups.