The review article provides up-to-date scientific information on the characteristics, classification and mechanisms of biological action of pro-, pre- and synbiotics in the digestive tract of ruminants. The literature sources of recent years on the influence of pro-, pre- and synbiotic supplements (when adding them to the diets of ruminants) on the metabolic processes in the body, intensity of growth, development and the quality of products obtained from domestic ruminant animals are systematized and analyzed. Emphasis is placed on the fact that the degree of metabolic and productive action of these diet supplements in ruminants is determined primarily by the qualitative composition, technology of production, method of storage and quantity added to fodder. It is noted that the main mechanism of pro-, pre- and synbiotics action when entering the digestive tract of ruminant animals is optimizing the composition of its microflora, strengthening the barrier functions of the rumen, reticulum, omasum, abomasum and intestine, as well as activation of interferon synthesis by blood leukocytes, stimulation of digestive functions and strengthening immune status. Also it is stated that the use of these fodder additives in the diet optimizes the quantitative and qualitative composition of the symbiotic microbiota of the digestive tract, has an immunostimulatory effect, activates metabolic processes and improves the productive qualities of ruminants.