Facial asymmetry refers to the fact that the left and the right sides of the face during movement or rest are not identical. This asymmetry can be produced by a range of factors, such as ones that are anatomical, physiological, neurological, psychological, pathological, or sociocultural. Interaction among these various factors is not uncommon and has the potential to confound scientific investigations. When focusing on asymmetrical facial movements and expressions--which is the issue of this article--one has to make sure to control for other types of asymmetries that could confound the observations. To ensure such control, adequate judgment and measurement techniques are necessary. Some of these techniques are briefly described in this article. Furthermore, some explanations for facial asymmetrical movements are considered. From a neuropsychological perspective, it is suggested that asymmetrical facial expressions have some relationship to the functional asymmetry of the brain. But social-psychological and cultural factors, it is argued here, may also play an important role in the phenomenon of asymmetrical facial expressions, although no definitive statements regarding the nature of their role are made at this point.