Cellulose and hemicellulose are the major structural carbohydrates present in forages and form between 30 and 60% of the forage component of ruminant diets. The complex network of structural carbohydrates and lignin generally leads to low digestibility and limits the efficient utilisation of forages by ruminants. This situation occurs in both developed and developing countries, and in the latter it is particularly pronounced as much of the forage component is based around the use of crop residues (Owen and Jayasuria, 1989). Because forage costs are significantly lower than those of other dietary ingredients, improving forage quality has been a major objective for many research programmes in both the developed and developing world. Improvements in forage quality have been sort through a number of different strategies. These have included conventional breeding techniques, which have included the integration of mutant genes, leading to the development of Brown Midrib varieties of maize and the use of chemical and biological additives. Enzyme supplements are commonly used to improve the nutritive value of feeds for non ruminants and as silage additives where they have been shown to improve silage fermentation, feed intake and performance. Recent work with ruminants has however focused on the use of enzyme supplements to improve feed efficiency by the use of “direct-fed” fibrolytic enzymes. This strategy involves the application of enzymes to feed at or only hours before feeding. These studies have yielded very variable production responses. For any new technology to be implemented widely, the responses achieved must provide an acceptable level of consistency and predictability. The current paper reviews developments in enzymology, production responses achieved and the effects on nutrient digestion.