Radioisotopic labelling is a technique for tracking the passage of a sample of substance through a system. The substance is "labelled" by including radionuclides in its chemical composition. When these decay, their presence can be determined by detecting the radiation they emit. Radioisotopic labelling is a special case of isotopic labeling.
For these purposes, a particularly useful type of radioactive decay is positron emission. When a positron collides with an electron, it releases two high-energy photons traveling in opposite directions. If the positron is produced within a solid object, it is likely to do this before travelling more than a millimetre. If both of these photons can be detected, the location of the decay event can be determined very precisely.
Strictly speaking, radiosotopic labelling includes only cases where radioactivity is artificially introduced by experimenters, but some natural phenomena allow similar analysis to be performed. In particular, radiometric dating uses a closely related principle.