Natural products like wool, leather or cotton are permeable to water vapor. Their complex fibrous structure makes it difficult to imitate this natural phenomenon by synthesis. This book discusses ways to obtain water vapor permeability by microporosity or through a hydrophilic structure. Various areas of application include the medical sector for implants and dialysis, the industrial sector for filtration or for processes requiring the slow release of substances, and the consumer sector for leather substitutes or performance textiles.