Proficiency of filamentous fungi for producing massive quantities of extracellular enzymes has promoted them attractive resources to be exploited in industrial manufacture of foreign proteins. A number of filamentous fungi are generally regarded as safe to be used, for example, in the food and feed industry. Numerous production bottlenecks are still a mystery, and overall production amounts of foreign proteins are still considerably lower than those obtained for native proteins. The less known attributes in protein production and secretion in filamentous fungi are discussed in the book, where the economically important fungus, Trichoderma reesei, occurs on the spotlight. The fungus has been exploited in food, animal feed, textile and pulp and paper industries, in biological control of plant disease, biodegradation of chlorophenolic compounds, and soil bioremediation. It has been harnessed also in production of calf chymosin and barley cysteine peptidase. Their production and secretory bottlenecks in the fungus are going to be revealed. Could we be able to get the fungus to manufacture effectively also therapeutic proteins in pharmaceutical industry?