The book examines the most important biochemical transformations of substances foreign to the organism (xenobiotics) that have entered it. The biotransformations of xenobiotics in humans and animals are presented in detail. The enzyme-catalyzed reactions of oxidation, reduction and hydrolysis (phase I reactions) and the condensation reactions for the formation of paired compounds (phase II reaction) are monitored. Special attention is paid to the biological consequences of biotransformations - neutralization, activation, occurrence of metabolites with carcinogenic effects. The biotransformations of different groups of xenobiotics are examined - industrial poisons, pesticides, drugs, household chemicals, etc. A significant place is devoted to the biotransformations of xenobiotics under the action of soil microorganisms as a factor in the self-purification of the natural environment. The biotransformations of xenobiotics in plants are also examined. The textbook is intended for students - biologists and chemists, but will also be of great interest to a wide range of readers: biologists, chemists, physicians, biochemists, pharmacologists, toxicologists, and specialists in environmental protection.