This document provides an overview of the psychology of groups. It defines a group as a collection of two or more interdependent individuals who usually feel, think and act together. It outlines the key characteristics of groups including interdependence, common goals, and shared norms and values. The document then discusses different types of groups and the stages of group dynamics including forming, storming, norming, performing, and adjourning. It also covers principles of group formation such as physical proximity, interaction, and similarity of interests/purpose. Finally, it identifies factors that influence group behavior such as sympathy, suggestion, and imitation.