The use of positron emission tomography (PET) has increased in oncology and in the assessment of head and neck tumours, where it is most useful for recurrent disease. It has good sensitivity and specificity for diagnosis and staging but is generally not necessary except in difficult cases. Quantitative measures of uptake on PET at diagnosis and after treatment do seem to have prognostic value independent of other information about the tumour and so PET may influence management. It also has a role in the identification of an unknown primary site and of synchronous primaries and metastases (often missed by other imaging). Fusion imaging with magnetic resonance (MRI) or computed tomography (CT) adds a new dimension with improved value for each technique.