Cellulose is the most abundant natural polymer on earth. It is the major constituent of cotton and wood, which together are the basic resources for all cellulose based products such as paper, textiles, construction materials, etc. Cellulose is also used as raw material for the production of blends, composites and nanocomposites which have a variety of different applications. In this chapter we review the main characteristics and properties of cellulose as well as its most promising potential applications emphasizing the use of composites reinforced with lignocellulosic fibers, nanocomposites reinforced with cellulose whiskers and bacterial cellulose nanocomposites. First, we start describing the structure and properties of cellulose at the molecular, supramolecular and morphological level. We present a review of cellulose whiskers, including the main processing techniques used for their preparation, as well as the influence of the processing conditions on the characteristics of such whiskers. We continue describing the manufacture of cellulose based blends, composites and nanocomposites. Composites reinforced with lignocellulosic macro-fibers as well as nanocomposites reinforced with cellulose whiskers and bacterial cellulose nanofibers are reviewed in this section. Finally, we present several applications for cellulose based composites and nanocomposites. This last section includes biomedical, optoelectronic and electrical applications as well as the use of cellulose for the preparation of high strength "nanopapers" and materials for packaging applications. © Springer-Verlag Berlin Heidelberg 2013.