This chapter discusses diseases of oral cavity, where oral epithelium may be affected by a number of conditions producing various lesions which may be due to local or systemic causes. Aphthous stomatitis is small, shallow, very painful ulcers with a clean base and surrounding erythema. Cauterisation of base is quite effective along with treatment of the cause. Behçet’s syndrome is characterised by ulcers in oral cavity, genital lesions and ocular lesions. Infective stomatitis may be due to viral infections such as herpes simplex or herpes zoster. Ranula becomes plunging ranula when it penetrates myelohyoid muscle. Malignant tumours may arise from lips, oral tongue, cheek, retromolar area, and mandible and histological types may be squamous or adenocarcinoma. Clinical features are ulcerative mass, raised margins, otalgia, and difficulty in opening the mouth with cervical lymphadenopathy Predisposing factors of malignant tumours are spirit, spices, smoking, syphilis, and sharp bad teeth.