This book synthesizes research on groups from the fields of social psychology and clinical psychology and encourages collaboration among group researchers.   Researchers in group psychology and group psychotherapy rarely consider each other’s work, despite their clear areas of common interest. This book demonstrates how these separate but related lines of research can be used to inform and enrich each other. Led by two past presidents of APA’s Society of Group Psychology and Group Psychotherapy, this book features expert contributors who examine the interface between social or organizational research on groups and clinical research, as well as the application of findings in each area. They reveal that different kinds of groups are far more similar than they are different, and illustrate how group psychology as a line of inquiry and practice benefits from improved dialogue among its domains. For example, there is growing literature on how individual personality factors such as attachment can affect group processes and outcomes in group psychotherapy. Similarly, research on expectations of social inclusion and ostracism in groups has a long history in social psychology, and there is great potential for these areas of inquiry to inform future research in both clinical and social contexts.