The β-glucanases and xylanases have been used as feed additives for many years and their ability to improve the performance of poultry has been demonstrated in numerous publications. Starch, proteins, and lipids easily degraded in the bird’s digestive systems, whereas the major parts of soluble and insoluble non-starch polysaccharides remain intact because of the lack of suitable enzymes. The positive nutritional effects achieved by the dietary supplementation of exogenous enzymes are proposed to be caused by several mechanisms. First, it has been shown that the antinutritive effects of viscous cereals such as barley, wheat, rye, oats, and triticale are associated with raised intestinal viscosity caused by soluble β-glucans and arabinoxylans present in those cereals. These problems are overcome by dietary supplementation of β-glucanases and xylanases. It is assumed that the ability of β-glucanases and xylanases to degrade plant cell walls leads to release of nutrients from grain endosperm. Another mechanism that has a positive influence on the nutritive value is the prebiotic effect achieved via the oligosaccharides produce and release. They formed during the degradation of cell wall carbohydrates by supplemental enzymes. In the gastro intestinal tract, the oligosaccharides derived from plant cell wall digestion that is resisting to the digestive enzymes, reach to the large intestine and support from proliferation of the beneficial microorganism. In conclusion, due to the complex structure of cereal grains, it has been shown that appropriate combinations of the different enzyme can obtain improved performance.