Nowadays computers are increasingly used for communication purposes and less for mere calculation. Userfriendly dialog design for non-computer professicial users is becoming an important research issue. The discussion has already shown that human-machine systems have to be studied as a whole: apart from the machine and its users they include the designers and those persons responsible for the system's application, as well. Computers just play a special role as one element in a highly complex communication network with several human agents linked in space and time. In order to characterize communication between humans and machines the concept of formal communication is introduced and related to natural communication. Communicating behaviour and its determining factors are represented by a model which is based on psycholinguistic concepts of communication and which uses high-level Petri net interpretations. Formal communication can be observed among humans as well as with machines; of ten it is caused by delegation. Programming of computer systems can be conceived as a special form of delegation. The view of computer systems as communication media with formal communicating behaviour permits an explanation of problems arising from computer applications, especially at the human-machine interface, and shows directions for future research.