Stable isotopes have become an essential tool to characterize and understand terrestrial and extraterrestrial matter. This chapter will briefly review the abundances of important light stable isotopes, demonstrate the link between abundance and atomic weight, introduce the notations and diagrams that are commonly used to report isotopic measurements, describe and partially explain the types of fractionation effects known to occur in nature, and direct the reader to more comprehensive sources of information on each subject. The special techniques needed to make accurate isotopic measurements gave rise to special notation for reporting stable isotope data, and these notations in turn gave rise to special diagrams that emphasize compositional differences and facilitate interpretation. Fundamental definitions are the isotope ratio R, representing the ratio of the abundance of a heavy isotope to that of a lighter, typically much more common isotope, and the isotopic fractionation factor α, representing the quotient RA/RB of the isotope ratios of two substances A and B. Under equilibrium conditions, lnα can theoretically vary linearly with 1/T at low temperatures or with 1/T2 at high temperatures, forming the basis for a standard graph. For practical reasons the ratio R is difficult to measure and inconvenient to report, so stable isotope abundances are usually reported as delta values (δ-values) that describe their deviations from a defined “standard” material. Thus, the most important diagram for data interpretation is the “δ-δ plot” where the δ-values of two coexisting phases are simply plotted against each other. In systems where two different heavy isotopes exist, two different delta values may be defined, each normalizing the abundance of one of the heavy isotopes to the common light isotope. In such cases, a very important diagram called the “three isotope” plot involves simply plotting these two different δ-values against each other for a given material, and the slopes of data arrays on such graphs can be used to distinguish ordinary “mass-dependent” fractionation (MDF) effects from “non-mass-dependent” fractionations (NMF). Numerous algebraic convolutions of the above definitions have been made, providing special definitions that can elucidate different phenomena. The processes that govern isotope distribution have become progressively better understood, yet recent studies show that these processes are more diverse than anticipated only ten years ago.