The clinical picture specific features as well as methods for the pathology prevention and treatment are depicted. It occurs when cells and tissues are damaged by radiation exposure or high doses of cytostatic drugs administration which makes impossible distinguishing “good” normal cells from “bad” cancer cells. As a result, an inflammatory process is mucositis. Oral mucositis is an inflammation of the mucous membrane lining the inner surface of the oral cavity. The cells of the oral mucosa are capable of rapid regeneration making them susceptible to the effects of chemotherapy drugs and radiation exposure. Oral mucositis is a frequent complication of radio- and chemotherapy of head and neck cancer. Its occurrence, risk factors, and mechanisms of development are described.