**Abstract**

Plant fibers have been shown to be highly sensitive to water molecules; this impacts the functionality of composites reinforced with these fibers, commonly known as bio-composites. This review aims to provide a comprehensive description of the behavior of plant fibers in the presence of water molecules in a liquid or gaseous state, as well as the different phenomena and mechanisms involved at the fiber scale and at the bio-composite scale via recent studies in this field. First, we will discuss the physical problem of sorption in polymers in a general way, and then we will focus on the case of plant fibers. Particular attention will be given to the adsorption kinetics of plant fibers and the models used to determine their diffusion parameters. In a second step, the effect of the incorporation of plant fibers in polymer matrices will be examined as well as the different factors influencing the diffusive behavior of biocomposites. In addition, the effect of hydro/hygrothermal aging on the mechanical properties of bio-composites will be discussed.

**Keywords:** plant fibers, bio-composite, hygrothermal behavior, aging, diffusion
