The geochemical applications of the traditional stable isotopes H, O, C, S and N and the non-traditional stable isotopes Li, Mg, Si, Cr and Fe are presented in this chapter. The standard isotopic notation, reference standards and isotopic distributions in some planetary reservoirs and the major Earth reservoirs are given for each isotope system. In each case the processes of equilibrium and kinetic fractionation are discussed and applied to high- and low-temperature (Earth surface and biological) systems and the fractionation factors listed. The utility of each isotope system as a tracer in geological and biological processes is discussed with applications to planetary differentiation, high-temperature igneous processes and low-temperature processes at the Earth’s surface in the terrestrial and aqueous environment and in living organisms. Stable isotopes are shown to be an important tracer for the evolution of life on Earth and the development of the Earth’s atmosphere.