Recent research on human factors in the man-machine communications interface is reviewed and discussed as it relates to the problem of establishing optimal system response times for the various kinds of tasks performed at the computer terminal. Optimal system response time is operationally defined, and the conflicting demands of system design, system performance, and computer users are outlined. A description of the task environment is provided, along with an analysis of the cognitive and behavioral characteristics of "general" and "naive" users, and a number of other considerations related to system response time are addressed. Response time measurement and specification are described as an integral part of the system design process, user-relevant measurements of response time are defined, and a rating scale for system response-time requirement specification is presented. An 82-item bibliography, a cited-author index, two figures, two tables, and appendices on various measures of system response and on the purpose and scope of user interface studies accompany the text. (JL)