Treatment and prognosis of malignant lymphomas depend on accurate staging and evaluation of clinical and histological features. Computed tomography (CT), magnetic resonance imaging and ultrasound are standard conventional imaging procedures for staging and, under certain circumstances, are supplemented by skeletal scintigraphy, biopsy from suspected areas of organ involvement and bone marrow biopsy. The determination of response to therapy, which is still the most important marker of prognosis [J Clin Oncol17 (1999) 1244], is usually performed at the end of therapy. However, this approach is being challenged (i) by the knowledge that the speed of tumour cell kill might be relevant particularly in high-grade lymphomas, and (ii) by the application of modern immunological treatments with monoclonal antibodies whose efficacy may last substantially longer than the time period they are infused. Nuclear medicine imaging with positron emission tomography (PET) has become an increasingly used imaging method for the evaluation of lymphoma patients and is capable of more sensitive and accurate staging, early prediction of response and prognosis after 2–3 cycles of chemotherapy, and even predicting the relapse probability after the end of treatment [J Nucl Med47 (2006) 1326]. The main advantages of the technique are based on the detection of tumour-specific metabolic cellular changes. Despite increased sensitivity in many but not all types of lymphoma, PET is not specific for tumour tissue and can be falsely interpreted. Additionally, the therapeutic consequences of the results of radioimaging are poorly defined despite the widespread use of the techniques, raising a number of unsolved problems. This article will review the use of PET and PET/CT in the staging and evaluation of response to therapy in malignant lymphoma and critically assess the pitfalls and advantages of this technique.