A vocational group in our society that uses a special language in their work was selected for a study of their language. The group consisted of air traffic controllers. This vocational group undergoes special training in connection with their work, which training provides them with knowledge and information together with a language and phraseology that enables them to clearly and concisely communicate with pilots and other controllers, The structure and function of this linguistic code was studied. The specified purpose of the language and phraseology of air traffic control is communication in which precise understanding is primary. Since a vocational group is a group in only a limited sense, this language is based on its use in the cognitive environment of flying -- an environment that contains concepts and values important in the physical environment of flying. Controllers, who are earthbound while working, must be familiar with this cognitive environment and relate to it easily. This research was conducted over a period of several months in the Air Route Traffic Control Center in Houston, Texas. Data were obtained primarily by observing and listening to the work of controllers. By being present during the classroom instruction of controller-trainees and in the control room while controllers were working, I was able to study the work and language of controllers in detail while it was in actual use. It was found that elliptical syntax in the phraseology of air traffic controllers was successfully utilized to provide brief, concise communication because both controllers and pilots operate in a situation where the background of knowledge and information are shared. I also found that while the goal of understanding in communication is highly successful, it is still possible on occasion to have a difference in interpretation, indicating that understanding in communication is not easy. A diachronic study of the language showed that, while occasional changes in the phraseology have occurred for clarification, more often changes have been in additions in procedure and phraseology owing to technological developments The language and phraseology now contain many additional acronyms and code word. Since the added procedures mean more information must be learned and utilized, it may be that acronyms provide a means of linguistic recoding that makes possible learning and processing of more information.