Trichoderma reesei (Hypocrea jecorina) is an efficient cell factory for protein production that is exploited by the enzyme industry. Yields of over 100 g secreted protein l(-1) from industrial fermentations have been reported. In this review we discuss the spectrum of proteins secreted by T. reesei and the studies carried out on its protein secretion system. The major enzymes secreted by T. reesei under production conditions are those degrading plant polysaccharides, the most dominant ones being the major cellulases, as demonstrated by the 2D gel analysis of the secretome. According to genome analysis, T. reesei has fewer genes encoding enzymes involved in plant biomass degradation compared with other fungi with sequenced genomes. We also discuss other T. reesei secreted enzymes and proteins that have been studied, such as proteases, laccase, tyrosinase and hydrophobins. Investigation of the T. reesei secretion pathway has included molecular characterization of the pathway components functioning at different stages of the secretion process as well as analysis of the stress responses caused by impaired folding or trafficking in the pathway or by expression of heterologous proteins. Studies on the transcriptional regulation of the secretory pathway have revealed similarities, but also interesting differences, with other organisms, such as a different induction mechanism of the unfolded protein response and the repression of genes encoding secreted proteins under secretion stress conditions.