Polymer composites are often submitted to severe humid environmental conditions during their service life. Moisture absorption by the polymer matrix induces various multiphysics effects that can significantly impair the functions and durability of the mechanical parts: weight gain, plasticization (reversible loss of mechanical properties, i.e. stiffness and ultimate strength), hygroscopic strains (volume change) and, eventually, damage. Most of these phenomena induced by the internal distribution of water also affect the moisture absorption process. This book is focused on the multi-scale coupled effects occurring during moisture absorption by polymer composites.