The potential for industrial enzyme products as animal feed additives has attracted substantial interest from feed manufacturers as a novel means of improving animal performance. Enzyme manufacturers have also targeted feed as an alternate outlet for their products, which have primarily been in the food, beverage, and detergent industries. Despite a history dating back 35 years or more, only recently has enzyme application been extensive and efforts in research intensified. The use of enzymes that degrade polysaccharides of the endosperm cell wall has become most prominent. The major cell wall polysaccharides are the β-glucans in barley and oats and arabinoxylans (pentosans) in rye, wheat, and triticale. In barley and rye particularly, the cell wall carbohydrates are prone to solubilization. The major enzymes are endolytic and achieve their beneficial effects by removal of diffusion constraints that interfere with nutrient absorption. Although most nutrients are affected, fat malabsorption may be severe in chicks fed unsupplemented diets containing barley or rye. Young chicks give the greatest response to enzyme-induced viscosity reduction; the response is much less evident in older birds or in swine. In addition to carbohydrases, renewed research in dietary phytase has occurred with the realization that phytases provide a cost-effective alternative to inorganic phosphorous in regions with dense populations and intensive livestock production, where excessive phosphorus in animal wastes is a national concern. Other enzymes may also be beneficial, including supplementary α-amylase (in young animals) and oligosaccharidases for feeds high in oligosaccharides; however, this has not been shown conclusively. Enzymes with desired activity and stability characteristics for feed applications will continue to be developed. Future directions for enzyme research may also involve genetic manipulation of the substrate to facilitate more complete enzyme degradation, as in the case of the fiber components of rye. Key words: Dietary enzymes, β-glucanase, pentosanase, phytase, feed, β-glucan, pentosan, phytate