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The Facial Action Coding System (FACS) Manual is a detailed, technical guide that explains how to categorize facial behaviors based on the muscles that produce them, i.e., how muscular action is related to facial appearances. It illustrates appearance changes of the face using written descriptions, still images, and digital video examples. Behavioral scientists, CG animators, computer scientists interested in pattern recognition programs, and other technicians and scientists use FACS in their professional work when they need to know the exact movements that the face can perform, and what muscles produce them. Working through the exercises of the FACS Manual may also enable greater awareness of and sensitivity to subtle facial behaviors that could be useful for psychotherapists, interviewers, and other practitioners who must penetrate deeply into interpersonal communications. 

FACS is a training manual, not necessarily easy reading, with lessons for detecting, performing, and categorizing facial movements. The manual does not discuss what the facial appearances described mean, except briefly in the Investigator's Guide. The FACS Manual enables the practitioner to recognize the elements of facial behavior that combine to create meaningful communications; FACS teaches the "alphabet" but leaves the considerable issue of semantics to other works. The FACS Investigator's Guide explains in general how to use FACS in scientific research, how it compares to other facial measurements, and what its psychometric properties are.