Chapter 10 335 the volume of hair on the head, leads to increased friction between fibers and a subsequent decrease in manageability. At the same time, the plasticizing effect of water alters the mechanical properties and is at the heart of certain styling-related issues. While typical consumers may not be familiar with these technical details, as already outlined there is certainly an awareness of water content influencing the properties of hair. In fact, to most, its presence is widely perceived as highly beneficial. When talking to consumers about hair care needs and desires, worries relating to “drying out” of hair are generally high on the list. Indeed, consumers may avoid or at least minimize the use of practices, products, and even ingredients that are perceived to induce this state. If this condition is realized, consumers seek “moisturizing” products that are perceived to “hydrate,” or “replenish a moisture balance” in an attempt to correct or restore hair properties. Such language is frequently recounted on product packaging or in advertizing material (often with a degree of embellishment) in an attempt to attract consumer attention and drive sales. However, as will be discussed, technical understanding of hair-water interactions and this consumer language do not equate. The primary goal of this chapter is to provide a comprehensive summary of existing knowledge concerning the interaction of water and hair. In undertaking this task, it is noted that much of this understanding arose from related studies involving hair’s commercially important cousin, wool, while much of that learning built on ideas and observations relating to the interaction of water with other important textile materials, such cotton and silk. As such, in some instances, concepts have been illustrated with references to this related literature. Nonetheless, recognition of the substantial effects of water on the properties of hair leads to contemplation of how other molecules may be used to purposely induce comparable and more permanent effects. Furthermore, an ability to manipulate this moisture content provides a means of influencing the mechanical properties of hair.