Lignocelluloses are structural materials in plants expected to withstand physical, chemical, and biological degradation. Nonetheless, biomass, with their content of approximately 70% sugar polymers is attractive materials for production of bioproducts. The purpose of lignocelluloses in nature and their application as sugar sources is hence contrasting to each other. The structure of lignocelluloses and their recalcitrance are matters lifted early in this chapter in order to facilitate the understanding of the factors in need of attention, involving their pretreatment. This discussion is followed by brief descriptions of physical pretreatments, that is, milling, irradiation, steaming, etc., physicochemical methods, that is, explosions with steam, ammonia, CO2, and SO2, chemical pretreatments by alkali, acids, gases, oxidizing agents, and organosolvs, and biological pretreatments by fungi and their enzymes. The different methods for pretreatment of lignocelluloses are compared in terms of their efficiency and from an economic point of view.