Our objective is to design computer systems which provide active computer support to users. “Active support” occurs when the computer works independently to assist the user, without explicit direction from the user. Such support would be very valuable to engineers. designers, planners, managers, and other professionals who work on complex problems.We argue that, for a computer system to provide active support, the system must build and maintain a model of the user's model of the problem. Such a model must be based on a valid descriptive theory of human problem‐working processes. We describe such a theory and show how it can be used in designing a computer system to provide active su