The Facial Action Coding System (FACS) is a comprehensive framework for categorizing human facial movements. Developed by Paul Ekman and Wallace V. Friesen, it identifies specific facial muscle actions, known as Action Units (AUs), that correspond to various emotions and expressions. FACS is widely used in psychology, animation, and artificial intelligence to analyze and interpret facial expressions. FACS provides a standardized method for researchers and practitioners to describe facial behavior objectively. By breaking down facial movements into measurable components, it allows for a better understanding of nonverbal communication and emotional states, facilitating studies in areas such as psychology, sociology, and human-computer interaction.