Radiotherapy in the treatment of head and neck tumors is most often used as an independent method or in combination with surgery and / or chemotherapy. These therapeutic methods in a multidisciplinary approach generally lead to favourable therapeutic response. During radiotherapy of this region, oral mucosa is inevitably covered within irradiated volume. Radical therapy is achieved with high doses of radiation, which usually results in development of undesired toxic effects, which, depending on the time of manifestation can be acute and late. Acute radiation toxicity occurs during or immediately after completion of performed therapy, and the late one several months or years after the completed treatment. The most common acute complications in the oral cavity are inflammation of oral mucosa, loss of taste, dry mouth and secondary infections. Late complications include radiation caries, trismus, and osteoradionecrosis. The aim of this paper was to present the effects and specificities of toxicity observed on oral cavity tissues after radiotherapy.