451 Chapter 14 The Effects of Aging on Hair—More Than Just Amount Michael J. Flagler, James R. Schwartz, Clarence R. Robbins, and Thomas L. Dawson The Procter & Gamble Company Introduction Most literature on how hair changes with age focuses on hair loss or alopecia, or changes in the aging hair follicle, the site where the hair is produced. While these are important areas of investigation that provide crucial insight into the biological mechanisms for some of the fundamental changes in hair as we age, it is equally evident that the hair fibers that emerge from our scalp exhibit significant changes as we age that have a great impact on the overall cosmetic properties of the hair. We have chosen to focus this chapter on the changes that occur to the actual hair fibers with age, as it is these changes that will ultimately be experienced by consumers as they age. Along with a critical review of the existing literature on hair aging, highlighting changes in hair pigmentation (graying), diameter, curvature, ellipticity, structural properties (stretching, bending, torsional rigidity), and lipid composition, we highlight the central knowledge gaps that need to be addressed for each of these parameters with age and provide both data and rationale for how changes to these hair properties will impact consumer perception of their hair as they age. In the final section of this chapter, we speculate with regard to the interdependence of these changes on the overall cosmetic properties of consumers’ hair. More work is required to determine