In the monograph, for the first time, the information on the mechanisms of digestion, the polypotentiality of the digestive enzymes as well as on the influence of some nature and anthropogenic factors on the enzyme activity has been systematized from the standpoint of modern trophology. The effect of various factors on the activity and the characteristics of digestive hydrolases and various components of enzyme-membrane complexes has been examined, as well as their role in the adaptation of enzyme systems that support digestive processes in fish has been described. Particular emphasis is placed on the influence of a electromagnetic field and electromagnetic storm on the activity of digestive hydrolases. The effect of temperature and pH on the enzyme activity of intestinal mucosa, chyme and enteric microflora in fish of different ecological groups as well as the effect of pH on the activity of the enzymes of the consumers, their potential preys, microbiota associated with preys and enteric microbiota in fish, which differ in their dietary habits and live in different water bodies was described. Adaptations and the evolutionary aspects of digestion in fish are discussed.