The monograph is a summary of current data on the use of microorganisms as chemical reagents. General theoretical problems and methodological approaches of this research area are discussed. The types of transformations of a wide range of organic compounds are considered in detail: steroids, hydrocarbons, alkaloids, antibiotics, vitamins, organic acids, etc. Particular attention is paid to the physiological aspects of microbial transformations and an assessment of the prospects for their industrial use.