Fluorine-18 labeled fluorine-2-D-deoxyglucose (FDG) is the most frequently used positron emission tomography (PET) probe but it has certain limitations when used in urological cancers. The introduction of co-registered PET and computed tomography (PET/CT) represents a major advance in technology and FDG-PET/CT has now become the new standard. The diagnostic performance of FDG-PET and PET/CT depends on the metabolic activity of tumor tissue, which is generally low in primary renal cell and prostate cancers and often in their metastatic deposits. In contrast, both seminomatous and nonseminomatous germ cell tumors are characterized by upregulated glucose metabolism with subsequently increased FDG uptake in tumor sites. Generally, the metabolic activity provides accurate information regarding the presence of a viable tumor, except in patients with residual mature teratoma. Although bladder cancer demonstrates sufficiently increased FDG uptake, primary tumors are difficult to identify due to the renal excretion of FDG. The accuracy of FDG-PET/CT in metabolically active metastases is generally higher compared to conventional CT except for identifying small lung deposits. With disease progression and subsequent de-differentiation of prostate cancer, castrate resistant disease is more likely to present with lesions that have increased glucose metabolism.