This paper details the characteristics of pure cellulose and wood fibers when associated with thermoplastic matrices for composite applications. Chemical modification of the cellulose is performed to allow a good compatibilization, and the most efficient compatibilizing agents must possess: (i) a function highly reactive with the OH groups of the cellulose and (ii) a non-polar chain with preferably a polymeric structure. Polypropylenes grafted with maleic anhydride are thus efficient agents. Smaller compatibilizing agents, especially if reacted with cellulose in swelling media, react with the bulk of the fiber and lead to dimensional stability. All treatments, even when performed with low degrees of grafting or small alkyl chains, significantly modify the hydrophilicity of the cellulose surface and play a role in a better wettability of the fiber by the matrix leading to improved adherence. The global mechanical properties are then improved, but the effect is preferably studied at the scale of a single filament composite. The morphology of the matrix in the vicinity of the non-treated fiber shows that, in some cases, the fiber acts as a nucleating agent involving the formation of a transcrystalline phase. Aging in moisture is generally detrimental to the mechanical properties. This phenomenon is limited by the chemical treatment performed on the fibers.