The advantages of PET are high sensitivity and specificity for the detection of local tumor relapse, especially in comparison with conventional imaging techniques. Furthermore, there is an additional value of FDG-PET for the detection of tumor manifestations associated with a tumor marker (e.g. serum CEA) increase without suspicious findings in other diagnostic procedures. The methodological advantage of a whole-body examination further improves the detectability of distant metastases. Thus, FDG-PET may induce a change in the therapeutic concept in recurrent colorectal cancer in approximately 30% of cases, concerning the indication for surgery as well as the use of multimodal surgical concepts. By means of the metabolic functional approach FDG-PET offers the possibility of therapy monitoring. This enables very early and more specific indication of response to preoperative therapies or of the presence of residual disease after surgical tumor resection or interventional tumor ablation of metastases or recurrences. PET using new tracer molecules demonstrates an additional advantage of the technique regarding the determination of the micromilieu of a tumor (vascularity, proliferation) compared with all other non-invasive diagnostic methods. This opens up new possibilities to optimize surgical therapy planning and prognostic evaluation.