The Effects of Aging on Hair–More Than Just Amount 484 properties must be elucidated to fully understand the full effects of age on hair handle or feel for females and males of the three major geo-racial groups. Conclusions Hair fibers exhibit significant changes upon aging which have a great impact on the overall cosmetic properties of hair. This chapter has provided a critical review of the existing literature on hair aging and highlighted the principal changes that occur to hair fibers with age, outlining the key knowledge gaps that need to be addressed. As we continue to develop a better understanding of how hair fibers change with age, it will be critical to examine the interdependence of such changes on overall hair assembly properties and consumer perception to enable the design of “anti-aging” cosmetic treatments that combat or reverse these effects. Summarized below are the key changes highlighted in this chapter, with special emphasis on hair diameter changes vsersus Age, as this parameter in particular has been well-studied and is likely to exhibit a strong impact to the consumer on the perception of aging hair. Hair Graying • Hair graying in Caucasians begins at ~25 years of age and increases with age to the point that by age 49 about 50% of the population has 50% gray hair. Unfortunately, comparable data for other geo-racial groups is lacking but the onset of graying has been proposed to occur later in those of African and Asian descent. • The perception of graying is impacted by hair color, with the perception of “any gray” occurring earliest in the darkest hair color and latest in the lightest hair this effect is reversed for “completely gray” hair, which is perceived to occur earlier for the lightest hair color and latest for the darkest hair. • There is conflicting literature on the interdependence of graying and hair curvature and diameter, and further studies are required to resolve this conflict.