Currently there are many unanswered questions concerning contouring a target with PET/CT in radiotherapy planning. Who should contour the PET volume-the radiation oncologist or the nuclear medicine physician? Which factors will contribute to the dual-observer variability between them? What should be taken as the optimal SUV threshold to demarcate a malignant tumor from the normal tissue? When the PET volume does not coincide with the local area CT findings, which portion should be contoured as the target? If a reginal lymph nodedraining area or a remote region is shown to be PET positive but CT negative, or PET negative but CT positive, how is the target identified and selected? Further studies concerning the relationship between PET/CT and the cancerous tissue are needed. The long-term clinical results showing an increased therapeutic ratio will finally verify the applicability of guidelines to contour the target with PET/CT in radiotherapy planning.