In applying cybernetic knowledge to the teaching process — which the author links up with the principles of programmed instruction — it is most essential to inquire into the problems involved in creating models. In a way, a programme of instruction can be regarded as a picture of the process during which the subject acquires the language, and as such it must have, — theoretically at least — a possibility of formally recording this picture by means of a mathematical or formally logical apparatus. If it is not so, then the programme does not meet the requirement of regeneration and individualisation of instruction, and one can hardly speak of applying cybernetics to the teaching process. Therefore the efforts of present day linguistics to arrive at an exact description of the language system are important for programming language teaching. Even though the greatest completeness and coherence have so far been achieved by means of the theory of generative grammars only in formally describing the speaker’s ability to make up certain — as far as the system is concerned — correctly formed sentences (linguistic competence), there are also attempts to create mathematical models of the process during which the speaker uses the language for communication (linguistic performance) and then of the process during which the subject acquires the language (language acquisition). Models of this type are of immediate importance for a general picture of the processes of language teaching which must be appraised just in connection with the cybernetic approach to instruction.