The introduction of Positron Emission Tomography (PET) and the use of the glucose analog F-18-Deoxyglucose (FDG) can help to improve the sensitivity of the diagnosis of lymph node metastases. Sensitivity exceeding 90% can be achieved when advanced imaging protocols and image reconstruction methods are used for PET. Superior staging information is obtained with PET as compared to morphological imaging methods for the most frequent tumor types. The accuracy of N-staging can be significantly improved by adding PET to the pretherapeutic diagnostic procedures. Limitations exist with regard to false positive results. Acute or chronic inflammation as well as unspecific reactions following radiotherapy may mimic tumor tissue.