The demand for environmental sustainability has resulted in a great interest in finding new materials that are biodegradable and environmentally friendly. Therefore, materials derived from natural resources are now being extensively studied. Preparation of novel biocomposites based on nanocelluloses has drawn specific attention. It is expected that cellulose nanocomposites will open new areas for applications in medicine, packaging, electronics, the automotive sector, construction and other areas. This article presents a new research field of bionanocomposites where different types of nanocelluloses are used as reinforcements in biopolymers. Isolation of cellulose nanofibres and nanowhiskers from different sources, and processing technologies for the composites, are described and discussed. The main difficulty when producing cellulose based nanocomposites is to disperse the reinforcement in the polymer matrix without degradation of the biopolymer or the reinforcing phase. This can be addressed by improving the interaction (compatibility) between nanofibres and the matrix and by using suitable processing methods. The study of alignment of the nanocelluloses by using magnetic field is discussed and the nanocomposites' mechanical properties, based on the findings from different studies, are presented. Finally, some examples of future nanocomposites are discussed.