Nuclear medicine is a multidisciplinary field that develops and uses instrumentation and tracers (radiopharmaceuticals) to study physiological processes and noninvasively diagnose, stage, and treat diseases. Particularly, it offers a unique means to study cancer biology in vivo and to optimize cancer therapy for individual patients. A tracer is either a radionuclide alone, such as iodine-131 or a radiolabel in a carrier molecule such as (18)F in fluorodeoxyglucose ((18)F-FDG), or other feasible radionuclide attached to a drug, a protein, or a peptide, which when introduced into the body, would accumulate in the tissue of interest. Nuclear medicine imaging, including single-photon emission computer tomography and positron emission tomography, can provide important quantitative and functional information about normal tissues or disease conditions, in contrast to conventional, anatomical imaging techniques such as ultrasound, computed tomography, or magnetic resonance imaging. For treatment, tumor-targeting agents, conjugated with therapeutic radionuclides, may be used to deposit lethal radiation at tumor sites. This review outlines the role of nuclear medicine in modern cancer therapy.