The history of the study of the symbiotrophic existence of plants, the origin of symbiont fungi, the production of their pure cultures, the role of endophytes in nitrogen fixation of plants and their participation in synthesizing activity are considered. On this basis, a microbiological theory of plant immunity was developed. Data on the use of the growth preparation Symbiont and the results of its use, which stimulates an increase in crop yields and increases the immunity of plants to microbial viral diseases, are presented. The monograph is intended for a wide range of scientists in the field of agricultural microbiology, biology and plant protection, teachers and students of agricultural universities, and also practical agronomists.