Lichen planus (LP) is a chronic inflammatory and immune mediated skin disease with characteristic clinical and histopathologic features. The skin and oral mucosa are the most frequently involved areas. Other mucous membranes (including the genitalia, esophagus, and conjunctiva) and skin appendages (e.g., scalp hair and nails) can also be affected. The etiology and pathogenesis of LP remain unclear. An autoimmune reaction in which CD8+-T lymphocytes attack basal keratinocytes leading to apoptosis of the cells has been favored. Various potential triggers, e.g. viral or bacterial antigens, metal ions, drugs or physical factors, could initiate the autoimmune process. Nonetheless, the role of the individual trigger factors is controversial. There is a significant association between lichen planus and infection with hepatitis C virus (HCV). The diagnosis of LP is based on the clinical presentation and should be confirmed by biopsy, if suspected. Histopathology is often conclusive, but in vesiculobullous cutaneous LP or erosive oral LP, direct immunofluorescence (DIF) studies can be an integral step in differentiating between LP and other diseases. With few exceptions, all forms of LP are treated with the same measures. Depending on the severity of the findings, topical and systemic treatment options are available.