In the proposed book, one of the most interesting phenomena in the animal world - symbiosis (coexistence) - is considered from a modern point of view. It includes three main types - mutualism (symbiosis in the narrow sense of the word), commensalism and parasitism. In addition, as a separate, fourth type of symbiosis, intraspecific parasitism, which occurs in some animals, is also considered. The publication is suitable primarily for biologists, doctors, veterinarians, agronomists and foresters, as well as for students of these specialties, but also for all those who are interested in the problems of zoology and biology in general.