Since radioactivity has been discovered as a natural phenomenon, only natural radioactive isotopes were available for the first studies. The basic concepts of radioactivity and radioactive indicators were developed using the natural radioac­tive isotopes; thus, these isotopes have mainly historical importance. From the late 1940s, artificial radioactive isotopes have been produced in cyclotrons, and later in nuclear reactors. Recently, the artificial radioactive isotopes are dominant in radio­tracer studies. Natural radioactive isotopes are used only in the environmental isotope transport modeling.