Unlike other muco-cutaneous lesions Oral lichen Planus(OLP) is often chronic and rarely undergoes spontaneous remission which make knowledge of the disease more important for oral physicians and dental practitioners. The etiology is not known, but there are several hypothesis involving genetic, infectious, psychogenic and autoimmune factors. Pathogenesis of OLP has been extensively studied but still the controversies surround the disease.Understanding the nature and behavior of OLP is also required in order to provide better treatment modalities. This book is an attempt to unify the proposed hypothesis and make understanding of the disease simple for not only oral physicians but also for dental practitioners.