Positron emission tomography (PET) using the radiolabelled glucose analog 2-[18F]fluoro-2-deoxy-d-glucose (FDG) is increasingly used for response assessment in patients with Hodgkin's disease (HD) and non-Hodgkin's lymphoma (NHL). These patients often present with a residual mass after therapy, but only a minority will relapse as most of these masses consist of inactive fibrosis. However, some patients have residual disease after first-line treatment and they can benefit from additional or early salvage therapy. Special interest for early, but accurate, assessment of response is growing accordingly. Conventional radiological techniques cannot differentiate between active tumoural tissue and fibrosis in these masses. In contrast, FDG-PET has the ability to differentiate between viable tumour and fibrosis and has been evaluated as an initial staging tool, for response assessment after completion of therapy and as a prognostic marker early during treatment. In this review, we will focus especially on the value of PET for response assessment.