(18) F-2-deoxy-d-glucose positron emission tomography (FDG-PET), combined with a multidetector helical CT (PET/CT) has emerged, in the past decade as one of the most important prognostic tool for lymphoma management. Besides proving as the only imaging technique able to recapitulate all the information yielded by the standard radiological staging and restaging, it provided new essential information for chemosensitivity assessment and radiotherapy planning. In lymphoma staging, functional imaging (FI) by PET/CT was shown to be more accurate than conventional radiological (anatomical) imaging to detect nodal and extranodal involvement, whereas in posttreatment restaging it showed a superior predictive value on treatment outcome. In Hodgkin lymphoma (HL), FI concurred to delineate a new paradigm of therapy in which PET is considered an essential tool to guide physician's choice on treatment intensity and modality. In fact, PET proved very useful for: 1) assessing chemosensitivity early during treatment to predict final therapy outcome; 2) managing a residual mass, detected by CT scan in up to two thirds of patients at the end of chemotherapy; and 3) planning radiotherapy in early-stage disease when conformal radiotherapy fields are used to spare toxicity to adjacent tissues. The early chemosensitivity assessment is the underpinnings of a new therapeutic strategy in HL, aimed at minimizing treatment-related toxicity while maintaining treatment efficacy. Several clinical trials are currently underway to test this hypothesis. In diffuse, large, B-cell lymphoma (DLBCL), PET/CT proved very useful: 1) in lymphoma staging, leading to upward stage migration in one third of the patients; and 2) to identify patients benefiting from consolidation radiotherapy for FDG-avid, single mass or limited-extension disease. Different to HL, the role of interim PET in DLBCL remains controversial. In follicular lymphoma (FL) preliminary studies PET/CT proved useful, in baseline staging to predict time to treatment in patients in which a watchful observation without treatment (watch and wait) was chosen as therapeutic approach treatment. In FL end-of-treatment PET/CT proved the most powerful prognostic tool to predict long-term treatment outcome.