Facial Action Coding System (FACS) is the most widely used and versatile method for measuring and describing facial behaviors. Paul Ekman and W.V. Friesen developed the original FACS in the 1970s by determining how the contraction of each facial muscle (singly and in combination with other muscles) changes the appearance of the face. They examined videotapes of facial behavior to identify the specific changes that occurred with muscular contractions and how best to differentiate one from another. They associated the appearance changes with the action of muscles that produced them by studying anatomy, reproducing the appearances, and palpating their faces. Their goal was to create a reliable means for skilled human scorers to determine the category or categories in which to fit each facial behavior. The FACS Manual was first published in a loose-leaf version with video or film supplements in 1978.FACS measurement units are Action Units (AUs), not muscles, for two reasons. First, for a few appearances, more than one muscle was combined into a single AU because the changes in appearance they produced could not be distinguished. Second, the appearance changes produced by one muscle were sometimes separated into two or more AUs to represent relatively independent actions of different parts of the muscle. (After all, facial muscles were identified and named by anatomists, not behavioral psychologists.)A FACS coder “dissects” an observed expression, decomposing it into the specific AUs that produced the movement. The scores for a facial expression consist of the list of AUs that produced it. Duration, intensity, and asymmetry can also be recorded.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.