Radiotherapy represents an important therapeutic modality in the treatment of lung cancer. Treatment response assessment after high-dose radiotherapy with or without concurrent chemotherapy using conventional imaging methods is limited since normal tissue appearance might resemble tumour recurrence very close. Positron emission tomography (PET) based imaging has been introduced in this situation with great enthusiasm and provides useful additional information on the biologic characteristics of the irradiated region, be it tumour or healthy lung tissue, provided some marginal conditions are taken into account. Furthermore, biologic imaging seems highly appealing for treatment guidance especially during treatment protocols including multimodality approaches with neoadjuvant intent. Treatment response might not only serve as a surrogate marker for pathological remission but for overall prognosis as well. Within this context, the optimal time point and the best parameter to evaluate remain issues of continuing debate. This review is aimed to give an overview of the current state of the scientific knowledge.