This paper discusses how user-centered design improves the performance of human-machine systems. The overall performance of such systems is a function of the individual performance of its two major components, the human user and the machine, and the performance of the interface between these two components. Accordingly errors and inefficiencies can result from the technological limitations of the machine, from the user's human limitations, and from limitations of the interface. In the vast majority of cases, system development effort is focused on advancing the technology, with little , if any, regard to the human-machine interface or the needs of the human users. Ignoring a major component of the human-machine system, the human, means that potential gains in efficiency and accuracy will not be realized. Worst yet, additional sources of errors and inefficiencies can be introduced by modifying the interface without regard to the user. In contrast, user-centered design takes into account th eneeds, characteristics, tasks, and limitations of the users. Based on an understanding of the user and his tasks, the designer can specify the functions the machine must perform so that the tasks are accomplished successfully by the human-machine system. This reduces some of the errors and inefficiencies that can be attributed to the machine component.