The first generalization in our country on the variety of forms and methods of identifying biological objects using keys. The author, a theoretical taxonomist, has been developing diagnostic problems for many years - a practically important area of ​ ​ biological systematics. For the first time, the publication fully presents modern types of keys, describes how to use them, what are their advantages and disadvantages. The book is equipped with a terminology dictionary and a brief outline of key theory. As a reference publication and methodological manual, it is intended for biologists of various profiles (zoologists, botanists, microbiologists), students, as well as a wide range of practitioners and persons interested in classification and identification problems.