Stable isotope geochemistry is a powerful petrogenetic tool in Earth Sciences that exploits the partitioning of isotopes between different phases or chemical species during geological processes. The technique can be used to interrogate a range of scientific problems from the origin and evolution of the solar system and the terrestrial planets to formation of rocks/ores to water and rock cycles, as well as modern-/paleo-climate and environment. Unlike radioactive decay, which is a nuclear process, variation in the isotopic composition of an element may arise due to differences in physicochemical properties of different isotopes because of their different atomic masses. An element may be suitable for stable isotope studies if it satisfies the following criteria: (i) has low atomic mass; (ii) has more than one non-radiogenic isotope; (iii) the mass difference between the isotopes is a significant fraction of the atomic mass; (iv) has interesting chemistry to fractionate the isotopes, e.g., the element may form bonds with a high degree of covalent character, or it may exist in more than one oxidation state (C, N, and S), or form a wide variety of compounds (O), or is an important constituent of naturally-occurring solids and fluids; (v) the rare isotope is sufficiently abundant to facilitate analysis.