Hemicellulase-aided bleaching is the first full-scale biotechnical application in the pulp and paper industry which truly exploits the unique specificity and safety of biocatalysts. Hemicellulases are used to modify the structure of xylan and glucomannan in pulp fibers in order to enhance the chemical delignification. This technology can be combined with various types of kraft pulping processes and bleaching sequences. The aims of the enzymatic treatment depend on the actual mill conditions, and may be related to environmental demands, reduction of chemical costs, or maintenance or even improvement of product quality. The technology is applied on the mill scale in several countries. This review describes the principles of the enzyme-aided bleaching, the composition of the fiber substrates, the basic enzymology involved, and the present knowledge of the mechanisms of the action of enzymes, as well as the practical results and advantages obtained on the laboratory and industrial scale.