This chapter offers information on the Computer Expression Recognition Toolbox (CERT), a computer vision-based system for recognizing facial expression, which uses detectors of facial action units. It also presents experiments analyzing spontaneous behavior with automated expression recognition to find new information about the coupling of movements such as eye openness with brows raised during driver drowsiness. The chapter tests the usefulness of CERT in various interactive applications that need automatic analysis, and discusses the Facial Action Coding System (FACS), a widely used method for coding facial expressions in the behavioral sciences. It concludes that research into facial expression dynamics which were previously infeasible by human coding will be enabled by the automated analysis of facial expressions.