By common usage phototherapy has come to mean the treatment of disease by ultraviolet (UV) or visible radiation either alone or with an added chemical. In dermatology phototherapy is generally restricted to describe treatment of skin disease with UV-B (290–320 nm) and the added chemicals are topically applied coal tar or anthralin. PUVA therapy refers to the treatment of skin disease with an oral or topical psoralen followed by exposure to UV-A (320–400 nm). Another term applied to this form of treatment is photochemotherapy. Thus in the context of this paper we will not consider the phototherapy of neonatal jaundice nor of tumours.