Accurate detection of head and neck cancer is crucial in patients' quality of life. The head and neck area consists of many complicated anatomical structures. Conventional imaging procedures such as CT and MRI provide much detailed information, but accurate estimation of the spread of cancer is still limited. Positron emission tomography (PET) using 2-deoxy-2-[18F]fluoro-D-glucose (FDG) is clinically useful in detecting head and neck cancer, providing accurate estimates of head and neck primary cancer especially in cases that are equivocal on CT and/or MRI. FDG-PET is able to show metastatic lymph nodes that may appear normal on CT and/or MRI. Further, whole body FDG-PET makes it possible to detect distant metastases. The clinical usefulness of FDG-PET in head and neck cancer is discussed in this review.