This chapter discusses the following mucous membrane disorders: pemphigus vulgaris, lichen planus, and Stevens–Johnson syndrome. Pemphigus vulgaris is an autoimmune disease that affects the skin and the mucosal membranes with blisters and erosions. Lichen planus is a cell-mediated immunological mucocutaneous disease; oral lichen planus may present with erosions, white streaks, or plaques in the oral cavity. Stevens–Johnson syndrome is an emergency dermatological condition in which an immunological hypersensitivity causes erosions and inflammation of mucosal membranes and the skin. As well as providing definitions of these diseases, this chapter discusses their etiology, typical symptoms, uncommon symptoms, demographics, natural history, complications, diagnostic approach, other diagnoses that should be considered, prognosis, and treatment.