NSP hydrolysing enzymes are used as feed additives especially in cereal based feed for young poultry and to some extent for piglets. The beneficial effects depend on various factors and vary for poultry between 0 and 9% with regard to weight gain. The mode of action of these feed additives is not well understood. The basis of action is the partial hydrolysis of NSP in the upper digestive tract leading to a decrease of digesta viscosity in the small intestine and to a softening of the so called cage effect. In addition to direct effects on nutrient digestion and absorption, morphological-histological modifications and metabolic responses of the intestinal tissues as well as modifications of quantity and composition of the intestinal microflora seem to be involved in the overall response to the NSP hydrolysing enzymes.