Oral Lichen Planus is premalignant condition, is a Diagnostic Dilemma. Our book provides a complete review on Etiological Factors, Pathogenesis, Clinical Features, Differential Diagnosis and all the Treatment Modalities. Oral cavity is thought to be "Mirror" or "Window" of the body because oral manifestations accompany systemic diseases and in many instances oral manifestations occur before systemic manifestations. Lichen planus is one among the various immune mediated disorders of oral mucosa. Lichen planus is a chronic mucocutaneous disease, which can affect oral mucosa, genital mucosa, scalp and nails.