The book is a publicly available introduction to the field of mathematics such as information theory, which is closely related to cybernetics and has a number of applications in communication technology, linguistics, biology, etc. In the third edition, the entire text was carefully viewed and numerous improvements were made to the presentation. Data on the theoretical and information characteristics of specific types of messages (written and oral speech, photo telegrams, television, etc.) are supplemented by results obtained in different countries during the 1960s.