Isotopes are nuclide having same atomic number but different mass number. They may be stable (nuclear composition does not change with time) or unstable (undergoes spontaneous decay). Radioactivity is a property of unstable isotopes to undergo spontaneous degradation of nucleus with emission of particles or rays to attain a stable conformation. There are nonionizing radiation and ionizing radiation. Types of ionizing radiations are alpha (α) radiation (having maximum ionization). Beta (β) radiation, γ radiation and X-rays. The γ rays and X-rays have great penetrating power and can pass through human body. Radioisotopes are used to study metabolic pathways, study half life of immunoglobulin, thyroid scan, radioimmunoassay and enzyme linked immunosorbant assay.