Positron emission tomography with integrated computed tomography (PET/CT) is increasingly used for the initial staging, final or even interim (mid-treatment) response assessment in malignant lymphomas. Extensive clinical experience has been gained with Hodgkin lymphoma (HL) and aggressive B-cell non-Hodgkin lymphomas, including diffuse large B-cell lymphoma (DLBCL), primary mediastinal large B-cell lymphoma (PMLBCL) and other subtypes, which are the subject of the present review. The use of PET/CT is now considered mandatory for baseline staging in these entities, providing more accurate information and obviating the need of bone marrow biopsy (BMB) at least in HL. PET/CT has been the long-standing “gold standard” for final response assessment. Furthermore, early interim PET evaluation provides valuable prognostic information in HL and DLBCL. In HL, it appears that treatment intensification with Bleomycin, etoposide, adriamycin, cyclophosphamide, vincristine, procarbazine, prednisone (BEACOPP)-escalated can improve disease control in patients with persistent PET positivity after two cycles of ABVD. However, there is no randomized evidence of survival benefit as yet. In contrast, regimens effective in overcoming the adverse impact of persistent PET positivity have not been yet described in DLBCL. The 2014 recommendations suggest the use of PET/CT for baseline staging and final response assessment in all [18F]fluorodeoxyglucose (FDG)-avid lymphoma subtypes, including the above named ones. The use of interim evaluation is not considered fully documented yet. The exact role of PET/CT in guiding treatment decisions has to be defined by ongoing and future randomized trials and evidence-based approaches are expected to become available in the near future.