The modification of surface properties of synthetic reinforcement fibres to modify composite interphase performance is mostly achieved by chemical functionalization techniques in aqueous media, and in some cases in organic media. In particular, surface treatments of glass fibres are carried out by the use of complex aqueous chemical systems, known as sizings, including one or more organofunctional silane coupling agents, a film former and other additives, i.e. cationic or non-ionic lubricants, anti-static agents, surfactants, wetting agents, chopping aids, and antioxydants). Natural fibres does not yet benefit from such a technological and scientific background. Thereby, many strategies of bulk and surface modifications are currently developed to implement natural fibres in composite materials applications. In this chapter, the different pre-treatments and functionalization treatments and related processes developed to modify natural fibres and interfacial properties in biocomposites will be exposed.