Fluorodeoxyglucose positron emission tomography (FDG-PET) is now considered the most accurate tool for the assessment of treatment response and prognosis in patients with Hodgkin lymphoma and aggressive non-Hodgkin lymphoma. This article discusses the potential and limitations of FDG-PET for response assessment in malignant lymphoma during chemotherapy (interim PET) and at the end of chemotherapy. Interim PET is used to predict the likelihood for a complete response at the end of such therapy. End-of-treatment PET aims to establish the completeness of response or the presence of residual viable tumor tissue. Until the results of ongoing clinical trials emerge over the next 5 years, interim PET should be considered investigational and should not be used for patient management outside of study protocols.