Since the introduction of (67)Gallium-citrate 30 years ago, nuclear medicine has played an important role in the evaluation of malignant lymphoma. During that time, several radiotracers were evaluated as potential alternatives for the diagnosis of lymphoma, but the introduction of (18)F-fluorodeoxyglucose PET (FDG-PET) marked a major turning point. FDG-PET took over most of the role of gallium, and is now an essential tool in the diagnosis of lymphoma. FDG-PET is increasingly being used for assessment of the tumor staging prior to treatment, for evaluating the response to treatment, and for monitoring the early reactions to therapy to predict the final outcome. FDG-PET has been shown to have more accurate diagnostic capability than conventional CT and MRI for distinguishing the tumor necrosis and residual masses frequently seen after therapy in lymphoma patients without any clinical and biochemical manifestation. Malignant lymphoma is the first disease for which FDG-PET was adopted as a tool for response assessment in the international standard criteria. However, lymphoma does not always display a clear high uptake, and there are some pitfalls in assessing the response to therapy. This review will highlight the most important applications of FDG-PET in lymphoma, focusing on the advantages and pitfalls of this imaging, and past and ongoing efforts to standardize the use of FDG-PET, particularly in response to assessment and therapy monitoring.