The benefit of FDG-PET in addition to standard work-up for carcinoma of unknown primary (CUP) and metastatic neck lesions has been widely described. However, most studies have been of retrospective nature with large heterogeneities in terms of workup standards and patient selection leaving several questions to be answered regarding the real value of PET in CUP. We here present an overview of the literature with focus on the current evidence of FDG-PET in detecting a primary in CUP and discuss the rationale of PET/CT in the diagnostic armamentarium of CUP.