Image segmentation for thoracic radiotherapy (RT) is one of the more difficult tasks in radiotherapy. In addition to varying morphological appearances of the primary tumour that can invade many critical thoracic structures, anatomical changes resulting from changes in tumour position and size during treatment delivery can make this task harder if re-planning is needed. Moreover, radiosensitive normal organs need to be delineated accurately and consideration needs to be given to their normal anatomical variables. This chapter provides an overview of considerations for RT planning computed tomography (CT), summarises the main challenges during image segmentation and gives suggestions to overcome them for lung cancer RT. Challenges during RT delivery are also covered along with considerations in delineating for adjuvant RT and for recurrent tumours.