Lichen planus is a chronic inflammatory mucocutaneous disease of unknown cause, rarely observed in young individuals primarily affecting women between the fifth and sixth decades of life. Although its pathogenesis is unclear, several studies suggest this is a persistent immune disorder where the basal keratinocytes are probably attacked by T lymphocytes. This book examines the potential pathogenesis of oral lichen planus, the clinical and histopathological criteria that need to be taken into consideration of the diagnosis of OLP and the association of oral lichen planus with systemic diseases and amalgam restorations.