Nowadays, natural fibres are considered as alternative bio-based reinforcements to synthetic fibres for composite applications. Indeed, natural fibres have many attractive advantages: renewability, biodegradability, good availability, low cost, low density, low abrasiveness, and they exhibit high specific stiffness and strength, in some cases as high as glass fibres, combined with interesting damping properties. Nevertheless, natural fibres also have some drawbacks as their flammability, low thermal stability, sensitivity to moisture and polar character lead to poor compatibility with hydrophobic polymer matrices. Therefore, modifications of natural fibres are required in order to target demanding technical and high performing applications. This Special Issue proposes to gather a series of articles dealing with the modification and control of annual plant fibres or wood properties for polymer composite applications. Indeed, composite properties can be controlled through various functionalization or impregnation treatments of plant fibres and wood, including chemical, enzymatic and physical treatments, such as ionizing radiations. Bulk and surface impregnation and grafting, coating, as well as modification of surface roughness, can be considered as strategies to enhance plant fibres, wood and resulting composites performances. Besides, relationships between specific biochemical and structural characteristics of plant fibres and wood and the efficiency of the modification and functionalization treatments are key issues (e.g., grafting regio-selectivity, interactions of functionalizing molecules with ligno-cellulosic substrates, …). The targeted properties could be flame retardancy, thermo-mechanical performances, improved resistance to water or other ageing conditions (non-exhaustive list). Modifications of plant fibres allowing their easier processing, or their incorporation into new performing polymer matrices, will also be considered.