45 3 Introduction Mastication is a learned automatic complex process involving interaction of hard and soft tissues in order to grind a food bolus prior to swallowing.1 It involves several nerves, muscles, and connective tissue structures, such as the tongue, masseter, temporalis and pterygoid muscles.2 Many factors can affect efficient mastication, such as maximal biting force, maximal mouth opening, tongue function, tongue force, and number of occlusal units.3 Loss of teeth, cavities, inadequate restorations, malocclusion, and periodontal diseases can negatively affect the chewing function.4,5 In patients with head and neck cancer (HNC), mastication may be disrupted due to HNC or cancer treatment, which can result in affected motor and oral functioning. Because of this compromised mastication, tougher foods are more difficult to process because they require a higher muscle force and more chewing cycles.6 Therefore, some patients switch their diet to softer foods, because the muscle force needed to break down food is too high.7 Treatment for HNC may consist of radiotherapy (RT), chemoradiotherapy (CHT), surgery, or a combination of these. Early-stage cancers are usually treated with either surgery or radiotherapy, while locally advanced cancers are treated with surgery followed by adjuvant radiation or chemoradiotherapy.8 RT damages all cells receiving a radiation dose, including normal tissue cells surrounding the tumor. This damage to normal tissues can result in acute or long-term damage. Acute effects include pain, mucositis, dermatitis, decreased saliva production, or edema. Long term damage can consist of dysphagia, fibrosis, edema, ulcers, vascular toxicity, or osteoradionecrosis.1,9,10 One of the most feared complications is osteoradionecrosis of the jaws. In order to prevent this, teeth can be extracted pretreatment, or hyperbaric oxygen treatment post-treatment can be prescribed. However, this cannot always be achieved, causing serious deterioration in dental health.1 Chemotherapy may show additional toxicities, for example by the enhancement of radiation-induced fibrosis of the muscles, edema or neuropathy.9 Surgery may require wide resections of one or multiple sub sites, including tongue, floor of mouth, or lower gingiva.11 Surgery may be combined with neck dissection or reconstruction of the tumor site by a tissue transfer. Impairments after surgery depend on volume of resection, tumor site and type of reconstruction. Patients may develop defects on soft tissues, bone, or dentition, which can lead to functional deficits in mastication, swallowing and speech. For example, tongue resection compromises lingual mobility and strength, and dental and mandibular surgery affect mastication.1 Although survival rates have improved over time, morbidity remains high.1 In order to determine the influence of HNC treatment on oral function, it is important to evaluate the masticatory performance in these patients.12