Cellulosic materials are used in the polymer industry for a wide range of applications including fillers, laminates and panel products, wood-polymer composites (WPC), polymer composites, alloys and blends, and cellulose derivatives called cellulosics. The derivatisation of cellulose has opened up tremendous marketing possibilities for the polymer processing, adhesives and pharmaceutical industries Commodity products made from waste cellulosic materials also show a promising future. Much effort has been devoted to the preparation of the wood-like composite materials (fabric laminates, plywood, particle board, medium density fiber board (MDF) that use wood waste or non-woody plants like sugar-cane bagasse. The study of polymer composites that contain cellulosic materials is also another important area of research. Cellulosic materials are rigid as compared to synthetic polymers. When cellulosics are used in polymer composites, it is of critical importance to reduce the free energy of the cellulosic surfaces to reduce agglomeration. Composites prepared under optimum conditions show excellent properties. The obtention of cellulose-reinforced materials, particularly thermoplastics, proceeds through modification of the polar cellulose surface by grafting with compatible thermoplastic segments. Natural cellulose has a melting point that is considerably higher than its decomposition temperature. Thus, natural cellulose cannot be used as a plastic material. However, cellulose is derivatised through the etherification or esterification, its properties are modified and it can then be melted and molded. Such cellulosic products are used in a large variety of applications. This report focused mainly on the synthesis of such derivatives. ln a first part, it is important to have a close look at the structure of native cellulose and the techniques used for its characterisation.