Filamentous fungi are dominant producers of a range of small molecule compounds such as organic acids e.g. citric acid, fatty acids as well as antibiotics and human therapeutics e.g. penicillin, cephalosporin lovastatin, taxol. They are producers of biosurfactants and polysaccharides and can be considered a food source in their own right e.g. mushrooms, single cell protein/biomass. Some fungi are notable enzyme producers which have been exploited for the production of cellulases, pectinases, laccases/ligninases, amylases, amyloglucosidases, phytase, proteases, microbial rennets, lipases and glucose oxidase. Fungal enzymes have applications in a wide variety of sectors such as in production of food and feed, pulp and paper, textiles, detergents, beverages, and biofuels. Fungi have an inherent ability to grow at high rates and to high biomass densities supported by low cost substrates in relatively simple fermenters. Filamentous fungi are renowned for high productivity characteristics making them naturally excellent producers of extracellular enzymes and metabolites. For example, the fungus Trichoderma reesei can produce cellulase at a level of 100g/L extracellular protein. They can also be used as candidate hosts for the expression of recombinant proteins.