The availability of isotopic varieties of the chemical elements has had a strong impact on many branches of science. For some procedures isotopic tracers make possible methods that are simply easier or more accurate or more convenient than other methods. More importantly, however, there are some processes, particularly those involving steady-state conditions, that before the advent of isotopic tracers were considered to be not accessible to investigation, but which can be studied with these tracers. In biological studies the radioactive tracers have been especially useful because external detection methods can be employed in noninvasive or minimally invasive studies.