Abstract: Automatic facial action units(AU) recognition can be applied in many areas such as behavioral science,man-machine communication,security and medical diagnosis,and has drawn much attention recently.This paper explains the conception and general procedure of automatic AU recognition,and introduces the main methods of feature extraction and AU classification.Besides,several typical AU-coded facial expression databases are presented,and the state of single AU,AU combination,AU temporal dynamics and AU intensity recognition is demonstrated.Finally,this paper summarizes the current difficulties of automatic AU recognition,and outlines the future development trend.