The term mucositis is coined to describe the adverse effects of radiation and chemotherapy treatments. Mucositis is one of the most common adverse reactions encountered in radiation therapy for head and neck cancers, as well as in chemotherapy, in particular with drugs affecting DNA synthesis (S-phase-specific agents such as fluorouracil, methotrexate, and cytarabine). Mucositis may limit the patient's ability to tolerate chemotherapy or radiation therapy, and nutritional status is compromised. It may drastically affect cancer treatment as well as the patient's quality of life. The incidence and severity of mucositis will vary from patient to patient. It will also vary from treatment to treatment. It is estimated that there is 40% incidence of mucositis in patients treated with standard chemotherapy and this will not only increase with the number of treatment cycles but also with previous episodes. Similarly, patients who undergo bone marrow transplantation and who receive high doses of chemotherapy have a 76% chance of getting mucositis. Patients receiving radiation, in particular to head and neck cancers, have a 30% to 60% chance. The exact pathophysiology of development is not known, but it is thought to be divided into direct and indirect mucositis. Chemotherapy and/or radiation therapy will interfere with the normal turnover of epithelial, cells leading to mucosal injury; subsequently, it can also occur due to indirect invasion of Gram-negative bacteria and fungal species because most of the cancer drugs will cause changes in blood counts. With the advancement in cytology, a more precise mechanism has been established. With this understanding, we can select and target particular mediators responsible for the mucositis. Risk factors such as age, nutritional status, type of malignancy, and oral care during treatment will play important roles in the development of mucositis. Many treatment options are available to prevent and treat this condition, but none of them can completely prevent or treat mucositis. More and more pathological methods are being developed to understand this condition so that better therapeutic regimens can be selected. Emphasis also should be made in assessing the patient's psychologic condition, particular depressive disorders. This is important because treatment with antidepressants will not only contribute in lifting depression but also reduces pain somatization. Although mucositis is rarely life-threatening, it will interfere with treatment of cancer to a great extent.