Radionuclides decay exponentially by emitting radiations to achieve stability. These decays are defined by disintegration rates and quantitated by mathematical equations, so that the quantity of a radionuclide at a given time can be assessed. The transient and secular equilibriums in sequential radioactive decay is detailed. Units of radioactivity are given in both standard and SI unis. Specific activity of carrier-free radionuclides is given. Half-life, mean life, and effective half-life is defined, and examples are presented.