Nuclear medicine is the branch of medicine that involves the administration of radioactive substances in order to diagnose and treat disease. The scans performed in nuclear medicine are carried out by a radiographer. This speciality of nuclear medicine is sometimes referred to as endoradiology because the radiation emitted from inside the body is detected rather than being applied externally, as with an X-ray procedure, for example.

For nuclear medicine scans, radionuclides are combined with other chemical compounds to form the radiopharmaceuticals. When administered to the patient, these radiopharmaceuticals target specific organs or cellular receptors and bind to them selectively. External detectors are used to capture the radiation emitted from the radiopharmaceutical as it moves through the body and this is used to generate an image. Diagnosis is based on the way the body is known to handle substances in the health state and disease state.

The radionuclide used is usually bound to a specific complex (tracer) that is known to act in a particular way in the body. When disease is present, the tracer may be distributed or processed in a different way to when no disease is present. Increased physiological function that may occur as a result of disease or injury usually results in an increased concentration of the tracer, which can often be detected as a "hot spot". Sometimes, the disease process leads to exclusion of the tracer and a "cold spot" is detected instead.  A large variety of tracer complexes are used in nuclear medicine to visualize and treat the different organs, tissues and physiological systems in the body.

The main difference between nuclear medicine diagnostic tests and other imaging modalities is that nuclear imaging techniques show the physiological function of the tissue or organ being investigated, while traditional imaging systems such as computed tomography (CT scan) and magnetic resonance imaging (MRI scans) show only the anatomy or structure.

Nuclear medicine imaging techniques are also organ or tissue specific. While a CT or MRI scan can be used to visualize the whole of the chest cavity or abdominal cavity, for example, nuclear imaging techniques are used to view specific organs such as the lungs, heart or brain. Nuclear medicine studies can also be whole-body based, if the agent used targets specific cellular receptors or functions. Examples of these techniques include the whole-body PET scan or PET/CT scan, the meta iodobenzylguanidine (MIBG) scan, the octreotide scans, the indium white blood cell scan, and the gallium scan.