This text reviews the mass of information concerning the ways in which individuals and groups differ from each other. Reviews of research findings and interpretations are provided on: physical appearance, performance and health; cognitive abilities; personality; and development across the life span. Extensive treatment of foundations (historical, measurement, research methods, biological, social, and cultural) is also provided. Both normal and abnormal behaviors are considered. The book provides an interdisciplinary focus, including material from all the behavior and natural sciences, not just psychology, sociology, or biology.