Nuclear medicine has been defined as the use of radionuclides (unsealed sources of radiation) in the diagnosis and treatment of disease. It is a predominantly diagnostic speciality. Of the two aspects of disease, disordered structure and disordered function, nuclear medicine or radionuclide diagnosis has been unique in consistently providing information about disordered function. The method has applications both in life ( in vivo examinations, often involving imaging radionuclide distributions in the body) and in the test-tube ( in vivo). The tracer principle has allowed insights in respect of both patient care and research across a broad spectrum of diseases. Although nuclear medicine is a specialty which continues to evolve, it already has an established role in health care, predicated on its ability to study function.