PET-CT is an exciting new imaging technology that simultaneously acquires detailed structural and functional imaging information. PET is having an increasing impact on the management of loco-regionally advanced non-small cell lung cancer with radiotherapy. PET combined with CT is much more accurate than CT alone in staging lung cancer and patients treated with radical radiotherapy or chemoradiotherapy have better outcomes because of superior patient selection. PET also has the potential to improve radiotherapy planning by minimizing unnecessary irradiation of normal tissues and by reducing the risk of geographic miss. PET influences treatment planning in a high proportion of cases and therefore radiotherapy dose escalation without PET may be futile.