This review aims to describe the characteristics of Oral Lichen Planus and factors that are involved in the initiation and progression of disease. Pathologic mechanisms behind this disease are discussed and available methods of diagnosis are suggested. Finally, complication of OLP and treatment options are explained. Lichen planus (LP) is a well known chronic inflammatory disease, affecting adults in 1-2% all over the world. There are six clinical forms of OLP: papular, plaque, reticular, bullous, atrophic and erosive. Apoptosis of epithelium cells is greatly elevated in OLP.