The DARPA PerceptOR program implements a rigorous evaluative test program which fosters the development of field relevant outdoor mobile robots. Autonomous ground vehicles are deployed on diverse test courses throughout the USA and quantitatively evaluated on such factors as autonomy level, waypoint acquisition, failure rate, speed, and communications bandwidth. Our efforts over the three year program have produced new approaches in planning, perception, localization, and control which have been driven by the quest for reliable operation in challenging environments. This paper focuses on some of the most unique aspects of the systems developed by the CMU PerceptOR team and the most immediate challenges that remain to be addressed.