Abstract: Digestion of cell wall fractions of fiber in the rumen is incomplete due to its resistance to microbial degradation. The ability to effectively utilise forage with high fiber content is important for the development of efficient animal husbandry systems. Exogenous enzymes able to modify forages before feeding are of considerable interest to researchers; important potential benefits include improved environmental outcomes. In situ assessment and in vitro gas production were used to evaluate rumen degradation characteristics and in vitro fermentation characteristics of forage. An in vivo study was used to evaluate the effects of exogenous enzyme application to ruminant forage. Recent studies have shown that adding exogenous enzymes to ruminant diets can increase animal production as a result of enhanced digestion of feed, although results have been inconsistent. This inconsistency is due to range of factors including enzyme properties, diet composition, enzyme addition rates, animal species and animal growth performance. One of the main problems for researchers is the lack of adequate biochemical characterization of different enzymes which leads to a poor understanding of their mode of action and effectiveness. The biochemical characteristics of enzymes are very important but enzymes are not always evaluated prior to use and there are no global standards for assessing the activity and quality of enzymes. This paper reviews the research on the development of exogenous fibrolytic enzyme use in animal production, mechanisms of activity on plant cell walls, animal responses, the factors influencing the use of exogenous enzymes in ruminant nutrition and problems associated with their use. The aim of the review is to help the development of improved, safer and more consistent enzyme additives for use in ruminant nutrition.