The central scientific goal of the ARPA Image Understanding Project research program is to investigate and develop ways in which diverse sources of knowledge may be brought to bear on the problem of interpreting images. The research is concerned with specific problems that arise in processing aerial photographs for such military applications as cartography, intelligence, weapon guidance, and targeting. A key concept is the use of a generalized digital map to guide the process of image analysis. The objectives, methodology, and current status of the research are described in this report. In the present phase of the program, the primary focus is on developing a road expert, whose purpose is to monitor and interpret road events in aerial imagery. Also described are the details of an innovative procedure for tracking road segments and finding potential vehicles in imagery of approximately 1-3 feet per pixed ground resolution. This work represents an important contribution toward achieving the indicated goals.