PET/computed tomography (CT) has been used for both diagnosis/ staging of cancer and guiding the cancer treatment planning process. PET-guided radiotherapy (RT) planning has been increasingly used to assist in determining the tumor locations so that therapy procedures can be focused on the tumor, minimizing damage to the surrounding tissue. However, incorporating PET/CT into the treatment planning process raises challenges in areas of immobilization, image registration, and target volume segmentation. This article focuses on the technical aspects of integrating PET/CT into radiotherapy planning and presents a general overview of the clinical workflow and challenges involved in the planning.