Despite recognized limitations, structural imaging with CT remains the standard technique for evaluating the response of lung cancer to both chemotherapy and radiotherapy. This evaluation has become increasingly important with the advent of neoadjuvant therapy before surgery. The high uptake of (18)F-FDG in most lung cancers and the demonstration that successful treatment reduces uptake have led to increasing enthusiasm for the use of PET and PET/CT to assess the therapeutic response. In this review, theoretic considerations and current evidence supporting the role of (18)F-FDG PET are discussed.