This chapter explores the complex biology and chemistry of human hair, offering a comprehensive examination of its structure, composition, and classification. It begins by contextualizing the cultural and historical significance of hair, before delving into its three major anatomical regions: the cuticle, cortex, and medulla. Emphasis is placed on keratin composition and bonding mechanisms, elucidating how these underpin hair strength, texture, and behavior. This chapter also examines melanin’s role in hair color and the biochemical processes behind graying. A detailed discussion follows on hair classification systems, including morphological and mechanical characteristics, fiber diameter, porosity, and density. The physiology of the scalp and its microbiome are analyzed in relation to hair health, with particular attention to common scalp disorders and aging-related changes. This chapter concludes with an overview of the hair growth cycle and the formulation science behind shampoos, conditioners, styling products, and colorants, linking cosmetic chemistry to hair’s unique biology.