This paper describes an active-camera real-time system for tracking, shape description, and classi"cation of the human face and mouth expressions using only a PC or equivalent computer. The system is based on use of 2-D blob features, which are spatially compact clusters of pixels that are similar in terms of low-level image properties. Patterns of behavior (e.g., facial expressions and head movements) can be classi"ed in real-time using hidden Markov models (HMMs). The system has been tested on hundreds of users and has demonstrated extremely reliable and accurate performance. Typical facial expression classi"cation accuracies are near 100%. 2000 Pattern Recognition Society. Published by Elsevier Science Ltd. All rights reserved.