Abstract : Various problems in voice communication systems found in operations underwater and in outer space are considered. Since the production of speech is basic to these problems, the major part of the article is devoted to factors affecting vocal production. Examples are given of special problems which arise when the voice communication situation involves highly restrictive environmental factors such as those encountered by deep sea divers, swimmers, and personnel within under-the-sea habitats or space capsules. In addition to the effects on speech of varying ambient pressures, changes to the voice caused by different gas mixtures are described. Several ways of making helium-speech more intelligible are presented. Problems with the electronic link between the talker and listener are discussed. It is pointed out that the effectiveness of communication systems can be increased by proper circuit discipline, training to speak clearly, and utilization of vocabularies specific to the immediate operation. The need for improved coordination among behavioral and engineering efforts to solve problems of communication in space and underwater environments is emphasized.