Wool, leather or cotton like most natural products are permeable to air and water vapor. The reason for this property is a fibrous structure consisting of chains of hydrophilic constituents such as cellulose or collagene. It is difficult to imitate this natural phenomenon by synthesis. Ways to obtain water vapor permeability by microporosity or a hydrophilic structure are discussed in this book (nearly 2,500 citations). Most commercial products permeable to water vapor substitutes for natural leather are named. Materials permeable to water vapor offer a wide range of use, the subsitution of leather was only the starting point. Other uses are in the medical sector for implants and dialysis, in industry for filtration or catalytic reactions or for processes demanding the slow release of substances. Further applications of this interesting chemistry will surely be developed in the future.