This book presents the results of comprehensive research of an inadequately studied class of secondary plant metabolites: phytoecdysteroids, which are structural analogs of the hormones of molting and metamorphosis of arthropods. The chemical structures of ecdysteroids isolated from plants of the genera Ajuga, Rhaponticum, and Silene have been established. Data on the physicochemical characteristics, reactivity, metabolism, and biological activity of these compounds are presented in this book. Considerations of the role of ecdysteroids in plants are expressed and data on their pharmacological properties are also given. Issues regarding the use of phytoecdysteroids in practical medicine and, accordingly, the technological aspects of deriving drugs on their basis and biologically active food additives of a fortifying type of action are considered as well. The book is intended for specialists in the fields of bioorganic and organic chemistry, biochemistry, biotechnology, and pharmacology. It is also relevant to scientists of various profiles and teachers and students interested in the problems of the chemistry of natural and physiologically active substances.