Based on the results of an empirical study of the process of troubleshooting in computer-controlled manufacturing systems, we highlight sources of difficulty encountered by expert maintenace personnel in acquiring and using information to diagnose faults. We focus on difficulties in collecting background information, testing hypotheses, making repair attempts, and generating beliefs about causes of failures. Recommendations are provided for how to make people more effective in the troubleshooting task by (i) changing management policies and practices at the plant level, (ii) modifying the process control hardware and software that already exists on the plant floor, and (iii) improving the design of new hardware and software for the next generation of process control systems.