Wool is renewable, natural fiber that has been used for millennia in textiles, but is at the same time, also a highly complex composite fiber that has potential for use in fiber-reinforced composites. Due to its structure, with some parts of the fiber being hydrophilic and some parts hydrophobic, it has a complex interaction with water and moisture vapor. In this chapter, moisture sorption mechanisms, both historical and current, are discussed in order to give an understanding of the effect moisture has on wool fibers. These effects including dimensional, morphological, and mechanical changes that occur upon wetting or drying of the fiber are also discussed and placed into context of how they may affect composite manufacture and performance. Moisture interactions with different types of wool composites are discussed. There are a number of modifications such as acetylation or plasma treatment that can be used to change the surface of the wool fiber affecting its hydrophilicity/hydrophobicity. These modifications may open up new ways of the manufacture and use of wool composites. There is also scope for designing wool composites that utilize the moisture sorption properties of wool to help buffer environmental moisture.