Three major contributions achieved by nuclear medicine in radiation oncology are (1) radioassays of various tumor markers, (2) radionuclide therapy, and (3) functional tumor imaging, which comprises about 50% to 60% of all nuclear medicine imaging. In general, radiolabeled ligand assays are more sensitive and less expensive than nonisotopic techniques such as the enzyme-linked immunoassay. Radioassays, particularly radioimmunoassays, are routinely used to detect and measure a wide variety of ligands for tumors. For an early and accurate detection of cancer, which is the key to successful treatment, there is an increased use of serum screening tests. Tumor markers are also used routinely to follow patients with various cancers. The use of unsealed sources of radionuclides in the therapy of cancer is largely unexplored except for iodine-131 for thyroid cancer, which has been used for more than 50 years. This is because of the unavailability of proper radionuclides and the expense involved with developing such radionuclides. Recently, however, there has been renewed interest in the therapeutic applications of newer radionuclides via systemic, intra-arterial, intratumoral, and intracavitary deliveries. Nuclear oncology is progressing with the development of both tumor-specific and nonspecific radiopharmaceuticals. There have been new approaches in imaging by means of single-photon emission computer tomography (SPECT), positron emission tomography (PET), and new hybrid SPECT/PET cameras capable of performing both types of imaging. All of these efforts will enhance cancer diagnosis, staging, follow-up, detection of recurrence, and differential diagnosis between necrosis versus viable tumor.