The major utilization of clinical PET is in oncology, and oncologic PET utilizes FDG as the radiopharmaceutical. FDG imaging demonstrates the increased metabolism by malignant cells compared to normal cells. The initial clinical application of FDG-PET was demonstrated in brain tumors, and the gradation of accumulation of FDG related to the degree of malignancy. Subsequent studies have documented the accuracy of FDG-PET in detecting and staging several different malignancies. Whole-body imaging has made a major impact on the ability of PET to document the distribution of malignancy.FDG-PET imaging is very accurate in determining if an indeterminate solitary pulmonary nodule is malignant and in staging lung cancer. The cost-effectiveness of PET has been demonstrated for these indications. Third-party payers have policies for paying for PET scans performed in the evaluation of solitary pulmonary nodules and in staging lung cancer. The preliminary data on the use of FDG-PET imaging in other malignancies supports its use in detecting liver metastases from colorectal cancer and differentiating fibrosis from recurrent tumor after therapy for colorectal cancer; staging the axilla in primary breast cancer; staging melanoma and lymphoma; and staging and detecting recurrence of head and neck cancer. The initial reports on the use of FDG-PET are encouraging in its use in musculoskeletal malignancy, ovarian cancer, pancreatic cancer, and thyroid cancer.