Incorporation of PET/computed tomography (CT) in radiotherapy planning plays a critical role in assisting gross tumor volume delineation for radiotherapy planning and delivery. As radiotherapy techniques evolve to become more conformal with the increasing use of intensity-modulated radiotherapy and stereotactic body radiotherapy, whereby sharp dose gradients exist between the target and adjacent normal tissue, accurate contouring of tumor targets is vital for the success of radiotherapy to achieve cure and locoregional control. This article outlines the integration of PET/CT into radiotherapy planning for head and neck, lung, and other solid tumors.