An intelligent mobile robot must be able to accept a mission statement and constraints, plan its actions, execute its plans, perceive and adapt to its environment, and report its successes and failures. In this paper we describe a modular system architecture for such a complex mobile robot system. On-board versus off-board processing is a key system-level issue. We have selected off-board processing because the anticipated computer quantity, size, power requirement, and lack of robustness made on-board processing impractical if not impossible. Our system includes a transportable command center and a computer-controllable M113 armored personnel carrier, our mobile robot. The command center contains communication and computer hardware necessary for receiving and processing robot motion and sensor information, and for generating and transmitting mobility and sensor commands in real time to the robot. All control and status data transmission, between the robot and the command center, is accomplished through microwave links using a wide band, auto-tracking antenna. Under development since 1982, this system has demonstrated the capability of mission and route planning with execution at 8 km/hr, obstacle detection and avoidance at 15 km/hr, autonomous road following at 24 km/hr, and a remotely managed route reconnaissance mission at vehicle speeds of up to 40 km/hr.