Early detection and staging of lung cancer is important in initiating rapid treatment and improving prognosis. Computed tomography (CT) and magnetic resonance (MR) imaging have a high resolution and are able to reveal structural abnormalities, but still have problems differentiating benign from malignant lesions. Lesion size is used as a distinguishing parameter but definite diagnosis still relies on invasive procedures. Positron emission tomography (PET) is based on imaging of biochemical processes in vivo. PET is unique by disclosing metabolic differences between benign and malignant disease, e.g. glucose utilization. Here, the role of PET in diagnosis and (re)-staging of lung cancer as well as monitoring of therapy response will be reviewed.