TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Process: In continuously hydrolyzing cellulosic biomass, a cascade of at least two reactors is provided in step (P), preferably additionally comprising (T) continually removing partially hydrolyzed cellulosic biomass from step (E), which has a viscosity, as measured in a Physica MCR 101 rheometer in a cup with a stirrer (FL 100/6W), of not more than 25 Pa-second, and transferring the same into a subsequent reactor, which can be operated at steady state; (E') performing further enzymatic hydrolysis on the partially hydrolyzed cellulosic biomass from step (E) in the subsequent reactor. The solid content of the cellulosic biomass, which is added to the reactor is at least 15%, preferably at least 20%, preferably at least 25%, further preferably at least 30%; further preferably 10% to 45%, further preferably 15% to 45%, further preferably 20% to 40% or 15% to 30%. The viscosity of the continually removed partly hydrolyzed cellulosic biomass as measured in a Physica MCR 101 rheometer in a cup with a stirrer (FL 100/6W), is not more than 10 Pa-second, preferably not more than 5 Pa-second, further preferably not more than 3 Pa-second further preferably not more than 1 Pa-second. Cellulosic biomass is lignocellulosic biomass and preferably comprises wood, annual plants, agricultural residues or waste, in particular bagasse or energy crops. It comprises more than 20% cellulose, preferably more than 50% cellulose, further preferably more than 70% cellulose. The cellulosic biomass is subjected to a pretreatment prior to the hydrolysis, where the pretreatment preferably is sulfite cooking and the cellulosic biomass preferably is lignocellulosic biomass. The reactor is a continuous stirred-tank reactor (CSTR). Extracellular or cell-membrane associated enzyme complexes, in particular mixture of cellulases and beta -glucosidases that can specifically hydrolyze the cellulose polymer into soluble glucose monomers is used in the hydrolysis step. The enzymes comprise cellulases, hemicellulases and/or beta -glucosidases. At least some of the at least partially hydrolyzed cellulosic biomass that is continually removed from the reactor is subjected to a separation step, where the non-hydrolyzed solids are at least partially separated from at least part of the hydrolysate, i.e. the liquid phase comprising hydrolyzed cellulosic biomass and enzymes, where the liquid hydrolysate is then mixed with cellulosic biomass, which is then introduced into a reactor, preferably a dedicated mixing reactor. A cascade of two or more reactors for hydrolysis is used, where at least some of the hydrolysate, i.e. the liquid phase comprising hydrolyzed cellulosic biomass and enzymes, from the first reactor is separated from the solid phase and is subjected to further processing, and where the solid phase from the separation is added to a subsequent reactor, preferably being smaller in size and/or volume than the first reactor, further preferably where an additional predetermined amount of enzyme is added to the subsequent reactor. The process is repeated for any subsequent pair of adjacent reactors in a cascade of three or more reactors for hydrolysis, where preferably each following reactor is smaller in size and/or volume than each preceding reactor, respectively. EXAMPLE - No suitable example given.