Speech, as a medium for communicating with computers, has several potential advantages, and its introduction may have far‐reaching implications for the evolution of information technology. Yet, despite the availability of speech I/O devices over the past few years, their introduction into day‐today use has been relatively slow. A human factors perspective helps to explain the predominance of the visual/manual interface, and the reasons for its preference over speech. This view reflects fundamental differences between the two modes of communication. However, human factors also suggest that much can be done to improve the usability of speech systems. Broad guidelines for the use of speech‐output devices are detailed, and include criteria for the assignment of output either to the visual or auditory channel. The human factors of synthetic speech are discussed, in terms of enhancing both its intelligibility and its acceptability to users. Performance measures of automatic speech recognition (ASR) technology must account for the sensitivity of such devices to extraneous factors. A description of contemporary ASR technology serves as a basis for the explanation of this sensitivity, and of the limited performance of such devices. Guidelines for the implementation of ASR technology are described that address both user behavior and the interaction of the device with its environment. Modality effects upon user behavior in a man‐machine dialogue are described.