This paper presents a real-time system for detecting and tracking humans from mobile vehicles. The system integrates human detection and tracking algorithms and employs depth information obtained from a stereo vision system. Depth information is used to limit the search space of the detector. We also present a simpler and faster variant of the popular Adaboost human detector. Experimental results demonstrate that integrating depth information vastly reduces the false detection rate compared with detection without stereo. Also, we show that integrating stereo enables the detection and tracking subsystem to operate in real-time.