PET/CT offers advantages over PET alone, which is limited by poor anatomic localization and CT alone, which provides morphologic data only. Retrospective fusion of separately acquired PET and CT images allows for potential fusion misregistration in the mobile head and neck between imaging sessions. Indications for PET/CT include recurrent neoplasm, tumor surveillance, and staging. This article will focus on recurrent head and neck neoplasm including, head and neck cancer, thyroid cancer, recurrent skull base tumor. PET/CT may change management in facilitating earlier detection of recurrence than is possible with conventional CT or MR imaging, in guiding biopsy, and in detecting second primary sites and distant metastases. Limitations of PET/CT include physiologic uptake, metabolically active tissue, and muscle contraction during uptake phase. PET/CT, however, is better equipped than is PET alone to mitigate these limitations by precisely localizing FDG uptake to anatomic structures. In addition, small lesions (< 1 cm) may be below scanner resolution and, therefore, a lower SUV (that is < or = 3), may suggest neoplasm. Recent treatment may result in false negative findings, especially when PET is performed within 4 months of radiation therapy. Finally, tumors of low metabolic activity (e.g., salivary gland tumors) may be prone to false negative results. In the future, PET/CT imaging will become more useful in staging head and neck cancer with improved scanner resolution. Development of specific tumor markers may allow for tumor-specific ligands that will increase sensitivity to head and neck neoplasia. Treatment targeting for radiation therapy is an application that is likely to become widely used.