Facial expressions convey a vast amount of information, but only recently have investigators begun to explore the precise details of what expressions are telling us about internal states, social behavior, and psychopathology. The Facial Action Coding System (FACS), which is a tool for comprehensively measuring facial expressions, plays a central role in this rapidly growing and exciting field. This volume represents the state of the art in research on facial expressions. Drawing from psychology, medicine, and psychiatry, the chapters address such key issues as the dynamic and morphological differences between voluntary and involuntary expressions; the relationship between what people show on their faces and what they say they feel; and whether it is possible to use facial behavior to distinguish among different psychiatric populations. The volume includes groundbreaking work on how the face reveals emotions, deception, psychopathology, and aspects of physical health. An essential reference for anyone pursuing research in facial expressions, this work combines classic papers with up-to-date commentary by the authors.