Lignocellulosic feedstocks include agro-industrial by-products, perennial grasses, vegetable and wood residues.They can be burned to produce heat and electricity and also be used to obtain liquid fuels (Naik et al., 2010).Wastes and by-products from agro-industrial processes such as coconut shells, rice husks, sugarcane bagasse, corncob and corn stover among many others, are abundantly produced in the world daily and have modest if any applications.These wastes and by-products are rich sources of cellulose and hemicellulose, which constitute important substrates in fermentative processes directed to biofuel production.However, as opposed to sugarcane juice or maize starch, these substrates are not readily available.The structural carbohydrates in the plant cell wall are wrapped up in lignin, which is an inert polymer that protects the plant and consequently constitutes an important barrier to fermentation.Therefore, a very effective way but not the only option to significantly increase biomass digestibility is lignin degradation or separation (delignification).The operation is aimed to increase the digestibility of constituent sugars through increment in gross material pore size (Sierra et al., 2008).It is challenging due to the recalcitrance of lignin and may require expensive chemicals and relatively high temperatures and pressures for acceptable reaction rates.Otherwise, at mild conditions (i.e.use of microorganisms or purified enzymes) it takes long times.Other ways to increase lignocelluloses digestibility include partial to total solubilization of hemicelluloses, and separation acetyl groups that link hemicellulose and lignin (Zhu et al., 2008).Crystallinity reduction of cellulose fibrils is sought because low crystallinity results in more reactivity; however, after delignification and sugar degradation with chemicals, an increased crystallinity is usually observed (Chang & Holtzapple, 2000).This increase is attributed to a preferential degradation of amorphous cellulose and less ordered crystalline forms during chemical pretreatment.A common method to obtain a significant reduction in crystallinity is sudden release of reactor vapor pressure.This operation is known as steam explosion.The operations aimed to turn lignocellulose digestible through either of the mechanisms described above are widely known as "pretreatments".They typically start with size reduction by chipping and grinding.In addition to being a rate-limiting step, a chemical pretreatment increases the cost of bioethanol production due to the high-energy requirements of heating and mechanical size reduction.Energy consumption during size reduction of wood may surpass 0.1-0.4MJ/kg, which is the required energy consumption to achieve sensible net energy output from wood to ethanol production (Kumar et al., 2009).Chemical separation of lignin and carbohydrates can be achieved through the use of acids, alkalies, and solvents, which promote selective solubilisation of either component.If acidic, carbohydrates solubilise; if alkaline, lignin degrades and solubilises (Mosier et al., 2005); if with solvents -widely known as organosolv pretreatment-carbohydrates solubilise (Zhao et al., 2009).Chemical processes may not be as selective as biological processes but may represent advantages related to required time, scalability, and process control.Biological delignification can be conducted using either microorganisms, which produce a set of enzymes that work synergically, or purified enzymes.The most widely used microorganisms are fungi from the Basidiomysetes family.Nevertheless, bacteria from Pseudomonas, Flavobacteria, Xanthomonas, Bacillus, Aeromonas and Cellulomonas strains can also decompose lignin and its derivatives.Biological lignin degradation can be conducted by culturing the microorganism in submerged, semisolid or solid cultures where enzymes such as lignin peroxidase, xylanase, laccase, and manganese peroxidase (among others) perform selective lignin degradation.www.intechopen.