The textbook is devoted to the presentation of fundamental questions of biochemistry on the chemical composition of living systems, the structure and properties of proteins, enzymes, nucleic acids, lipids and carbohydrates, as well as vitamins, hormones and mineral compounds. The book includes general principles of metabolic processes, the exchange of proteins, carbohydrates and lipids, the exchange of nucleic acids, water-salt metabolism, their regulation and relationship. Here, for the first time, the mechanisms and factors of stress states in animals and humans are considered. The book is written in light of modern advances in molecular biology and medical biophysics regarding the use of free radicals and their non-radical reactions in structural and kinetic studies of proteins and nucleic acids. The book is intended for university students specializing in biochemistry and molecular biology.