To understand why there are oral complications of cancer therapy, we must first make the distinction between treating cancer of the head and neck and treating other types of cancer. While the surgical and radiotherapy management of head and neck cancers has localised effects on the oral cavity, cancer therapies involving drugs such as chemo- or immunotherapy have systemic effects, which can cause a number of side effects, including those specific to the oral cavity (Table 80.1). Both radiotherapy and chemotherapy destroy rapidly dividing cells. The oral cavity is lined by oral mucosa, which is highly susceptible to radiotherapy and the systemic effects of chemotherapy.