Anaerobic fungi are the inhabitants of the digestive tract of herbivorous mammals, ruminants as well as non-ruminants. One of the major characteristics of all anaerobic fungi examined thus far, is their production and secretion of a range of polysaccharide-degrading enzymes, including cellulases, xylanases and glucoside-hydrolases. The cellulolytic enzymes of the anaerobic fungus Neocallimastix frontalis have been shown to possess a high activity. Therefore anaerobic fungi and/or their enzymes could be interesting for many biotechnological applications including saccharafication of lignocellulosic residues, production of polysaccharidehydrolysing enzymes. This review summarizes the present knowledge of anaerobic fungi with special emphasis on their cellulolytic and xylanolytic enzymes. Further, a comparison with aerobic fungi is made.