There are a large number of natural fi bers that have the potential to replace synthetic fi bers made from glass, carbon, and polymers as the reinforcing phase in composites. Interest in these fi bers has increased, as they offer a sustainable, readily available, high specific modulus option. The high modulus of cellulose crystals makes them attractive as a reinforcing phase in composites. These cellulose crystals are the key structural component in natural fi bers, but the complex microstructure of the fi ber means that the full value of the cellulose crystal modulus cannot be utilized. Moreover, the surface of natural fi bers can have varying chemistry depending on the pretreatment of the fi ber and the degree to which lignin and other hydrophobic materials are removed from the fi ber. In addition, cellulose swells in contact with water, and this can degrade the properties of the fi ber and composite. A better understanding of the surface properties and techniques to control the composite interface are required if natural fi bers are to fulfi ll their potential.