Oral cavity includes mouth, tongue, lips, palate and tonsils. The chapter highlights the diseases or lesions of the mouth, lips, tongue, palate and tonsils are described. It also covers etiology, clinical features, diagnosis, investigation, treatment and complications of these diseases. Tonsil is an ovoid shaped, subepithelial aggregate of lymphoid tissue situated in lateral wall of oropharynx between anterior and posterior pillar. Acute tonsillitis, chronic tonsillitis, tonsillectomy, etc. are covered in this chapter. Neoplastic lesions of oral cavity including benign, malignant, Premalignant, are explained too. Clinical features, investigation, tumor, node and metastasis (TNM) staging, and treatment of oral cancers along with how to clinically examine oral cavities are described in this chapter.