In the monograph, the results of scientific investigations executed by researchers of the Institute of Bioorganic Chemistry of tne National Academy of Sciences of Belarus (Minsk, Belarus) and the Department of Organic Chemistry of the Wageningen Agricultural University (Wageningen, The Netherlands) are presented. The book covers structures, classification, isolation, spectroscopic determination, biosynthesis, metabolism, physiological mode of action, practical applications in agriculture, etc. of a new class of plant hormones - brassinosteroids.