Isotopes are simply atoms of the same element that contain different numbers of neutrons in their nucleus. While some atoms undergo radioactive decay, the species we investigate are all naturally occurring and stable — that is they do not undergo decay. The difference in number of neutrons leads to a very subtle difference in atomic weight among isotopes of a given element. For example carbon may have an atomic weight of 12 or 13 and although each carbon isotope behaves similarly chemically, the difference in mass leads to small differences in thermodynamic and kinetic properties. Therefore during processes such as evaporation, metabolism, and simple exchange reactions, the heavy and light isotopes (and the molecules in which they may reside) tend to be partitioned preferentially into different products. This fractionation process ultimately leads to naturally occurring fluids such as waters and natural gases developing unique ratios, or signatures, of stable isotopes. This forms the basis for the Isotope Fingerprinting technique. The strength of this technique is because we examine the molecule of water, or methane, itself, and not a proxy tracer that must be added to the system.