Lung cancer is a common type of cancer and it represents a substantial social and economic burden. Despite intensive efforts for lung cancer screening and follow-up, it is still the leading cause of cancer-related death. With intensive systemic treatment with chemotherapy, overall response rates are far from optimal and patients experience significant treatment-related toxicity. In order to identify non-responders, molecular imaging with 18F-fluoro-deoxy-glucose positron emission tomography/computed tomography (FDG-PET/CT) can be used. In this thesis, the results of FDG-PET/CT forearly in-treatment response in non-small cell lung cancer (NSCLC) are described.