Trichology is defined as the study of hair, including its general biology and evolution. As early synapsids and sauropsids shared common placodes that grew scales, they evolved different integuments that would eventually define the epidermis of contemporary species. Mammals derived from the synapsids and gradually evolved in the form that we know today as endothermic vertebrates that bear mammary glands and hair, the latter integument having evolved first. All mammals, from the egg-laying platypus to the placental blue whale, grow hair, although some may appear bare-skinned. Each individual hair is tiny and appears insignificant, morphologically speaking, but its biology, including its functions, is impressively complex. This chapter introduces trichology and presents a general overview of the literature describing the evolution of hair, the specific case of human body hair, the biology of hair morphogenesis, and the numerous biological functions of hairs. It focuses on the biology and functions of the sum of hairs, termed ‘pelage’, including moulting, colouration, and thermoregulation.