**1. Introduction**

The depletion of natural resources, the pollution caused by industry, and the current environmental challenge have led researchers and industrialists to explore new ways of providing green and renewable resources, such as biomass. Among these promising natural resources in the field of composite materials are plant fibers, also known as cellulosic or lignocellulosic fibers. Indeed, the integration of these fibers into the field of composite materials as an alternative to synthetic fibers continues to attract much research and application interest. This is due to their neutrality with regard to CO2 emissions into the atmosphere, but also to the fact that these fibers have specific mechanical and thermal properties. In addition, these fibers have other advantages, such as their low cost, their biodegradability, and the fact that they are renewable.

However, the hydrophilic nature of these plant fibers and the diversity of their properties represent limitations for their use as reinforcement for composite materials. This is particularly true of hydrophobic matrices, which raises questions about the compatibility between the reinforcement and the matrix on the one hand, and the durability of the bio-composite on the other, especially when the latter is exposed to changing hygrothermal conditions.

This chapter will comprehensively address the hydro/hygrothermal behavior of plant fibers and their influence on the durability of bio-composites. 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 bio-composites. In addition, the effect of hydro/hygrothermal aging on the mechanical properties of bio-composites will be discussed.

Before starting this chapter, it is necessary to define some basic knowledge that are essential for understanding the physical phenomena that will be presented.
