Lichen planus (LP) is a common oral disorder which may represent the manifestation of a mucosal reaction to a variety of aetiological factors. Of the many immunological changes described in LP several may he epiphenomena although a cell‐mediated immune response to unidentified antigens or antigenic changes clearly is involved. Associations with drugs or systemic disorders are well‐defined in some, patients but may, in others, be aleatoric, Finally, the prognosis of oral LP not only differs from that of cutaneous LP but there is little doubt that in a few instances LP is a premalignant condition.