Applications of xylanases can be found in the food, feed and pulp/paper industry. Filamentous fungi are particularly interesting producers of this enzyme from an industrial point of view, due to the fact that they excrete xylanases into the medium. Furthermore, xylanase levels from fungal cultures are generally much higher than those from yeasts or bacteria. In addition to xylanase, fungi typically produce several accessory xylanolytic enzymes, which are necessary for debranching substituted xylans. An important factor for efficient xylanase production is the choice of an appropriate inducing substrate, either insoluble or soluble, as well as optimization of the medium composition. In addition, the substrate can influence the concomitant formation of cellulolytic enzymes in certain organisms. Bioprocess parameters, that can affect activities and productivities of xylanase attained in a fermentation process, as well as the concurrent formation of cellulases, include the pH, temperature, or agitation. Xylanase activities produced by different organisms, including filamentous fungi and yeasts, are compared for both submerged and solid-state fermentations. When available, data on the concurrent formation of cellulolytic enzyme activities are included. On an industrial scale, xylanases are produced mainly by Aspergillus and Trichoderma spp. A list of commercially available xylanases and their potential applications is given.