Molecular imaging with positron emission tomography (PET) has dramatically changed the management of patients with lung cancer who are treated with radiotherapy. Uptake of the most widely used PET radiopharmaceutical (18)F-fluorodeoxyglucose identifies lung nodules or intrathoracic lymph nodes as likely to be malignant and frequently identifies previously unsuspected sites of malignant disease outside the thorax. Patients with non-small cell lung cancer, especially those with apparently more advanced locoregional disease to start with, are often upstaged, and this has a profound effect on their subsequent management. Better patient selection, primarily by excluding patients with PET-detected advanced disease, and better targeting of radiotherapy to avoid geographic miss have contributed significantly to improvements in outcome in recent series of patients treated with definitive chemoradiation. Advances in motion management with 4-dimensional PET/computed tomography and research into sequential imaging during treatment to permit response-adapted therapy hold promise for further improvements in treatment outcomes. Research involving novel PET tracers that can characterize biological properties of tumors, such as proliferation or hypoxia, may help develop more personalized approaches to patient management in the future.