The principal functions of the skin include protection, excretion, secretion, absorption, thermoregulation, pigmentogenesis, accumulation, sensory perception and regulation of immunological processes. These functions are all affected by the structural changes in the skin with ageing and, after middle age, most functions are reduced, some by as much as 50-60%. The physiological changes associated with these reductions include impairment of the barrier function, decreased turnover of epidermal cells, reduced numbers of keratinocytes and fibroblasts, and a reduced vascular network particularly around hair bulbs and glands. These changes result in fibrosis and atrophy, and decreases in hair and nail growth, vitamin D synthesis and the density of Langerhans cells. Production of epidermal thymocyte-activating factor, which enhances the T-cell response, is reduced leading to a decrease in the immune response; there is also a decreased functioning of Meissner's and Pacinian corpuscles. An increased generation of free radicals is observed. Certain environmental factors, particularly exposure to sun, accelerate the ageing of skin and are important in cutaneous carcinogenesis.