Lichen sclerosis is a chronic inflammatory skin disease that causes substantial discomfort and morbidity, most commonly in adult women, but also in men and children. Any skin site may be affected (and, rarely, the oral mucosa) but lichen sclerosus is most common in the anogenital area, where it causes intractable itching and soreness. In children, the disorder may be confused with changes seen in sexual abuse. Progression to destructive scarring is common. There is increased risk of developing vulval cancer, and there are links with penile cancer. Patients should be kept under long-term review. Lichen sclerosus can occur without symptoms, and the exact prevalence is uncertain. It occurs most commonly in women at times of low sex hormone output. The underlying cause is unknown, but there seems to be a genetic susceptibility and a link with autoimmune mechanisms. The wart virus and the spirochaete borrelia have been suggested but not substantiated as infective triggers. The Koebner phenomenon is known to occur (lichen sclerosus occurs in skin already scarred or damaged), so trauma, injury, and sexual abuse have been suggested as possible triggers of symptoms in genetically predisposed people. The treatment of choice for anogenital lichen sclerosus is potent topical corticosteroid ointment for a limited time. Circumcision may be indicated in men, and surgery may be considered in women, to relieve effects of scarring or to treat coexisting carcinoma. Current research aims to identify a treatable cause of lichen sclerosus, to identify patients at risk of scarring and of malignant disorders, and to find target pathways for therapeutic intervention.