Radioisotopes are radioactive isotopes of an element that have unstable nuclei and emit radiation to transform into a more stable form. They can also be defined as atoms that contain an unstable combination of neutrons and protons, or excess energy in their nucleus. Radioisotopes can be produced naturally or artificially.

Radioisotopes have many useful applications in medicine, industry, and scientific research. In medicine, radioisotopes are used for diagnosis, treatment, and research. Radioactive chemical tracers emitting gamma rays or positrons can provide diagnostic information about internal anatomy and the functioning of specific organs, including the human brain. This is used in some forms of tomography: single-photon emission computed tomography and positron emission tomography (PET) scanning and Cherenkov luminescence imaging. Radioisotopes are also a method of treatment in hemopoietic forms of tumors; the success for treatment of solid tumors has been limited.

In industry, radioisotopes are commonly used in industrial radiography, which uses a gamma source to conduct stress testing or check the integrity of welds. They are also used by industry for gauging (to measure levels of liquid inside containers, for example) or to measure the thickness of materials.

Radioisotopes are also widely used in scientific research and are employed in a range of applications, from tracing the flow of contaminants in biological systems to determining metabolic processes in small animals. They are also used on behalf of international nuclear safeguards agencies to detect clandestine nuclear activities from the distinctive radioisotopes produced by weapons programs.

Overall, radioisotopes have many important applications in various fields, and their use continues to be an active area of research and development.