Each year, head and neck cancer comprises more than 550,000 cases and contributes to 380,000 deaths, worldwide. Radiation Therapy (RT) is used for head and neck cancers as definitive, adjuvant, or palliative treatment. The most important advantage of RT, compared with surgery, is function preservation; however, normal tissue complications occur during (acute complications) or after (subacute and chronic complications) RT in some cases. Late adverse effects of RT cause serious oral complications, such as xerostomia and hyposalivation, trismus, dental caries, and osteonecrosis, resulting in decreased quality of life. The aim of this chapter is to review the aetiology and pathogenesis of oral complications (with particular focus on dental demineralization and osteoradionecrosis) through previous literature and our clinical experience. This chapter also discusses prevention and treatment of those oral complications