Hair weathering is the deterioration of the hair shaft from root to tip resulting from a range of cosmetic and environmental factors. Cosmetic practices such as combing, brushing, braiding, weaving, hair extensions, straightening, waving, perming, and dyeing, as well as environmental factors such as exposure to UV light, can induce structural damage to the hair fiber. The cuticle becomes raised and porous, exposing the cortex to further damage. A decrease in hair shine, elasticity, and strength is observed, ultimately leading to hair breakage. Part 1 of this series reviews hair structure, including race and age variations, as well as the pathogenesis of hair weathering.