The stable isotopes of hydrogen and oxygen of water molecule, environmental radioactive isotopes 3H, 10Be, 14C, 26Al, 32Si, 36Cl, 39Ar, 53Mn, 59Ni, 81Kr and 137Cs, nuclear explosion-produced 3H, 36Cl, 85Kr, 129I are some of the isotopes which have played a vital role in understanding the movement of water in the hydrological cycle.