The stable (non-radioactive) isotopes commonly used in archaeological analysis and their distributions in the natural environment are described in this chapter. The properties of the stable isotopes of an element are almost identical, but some processes favor one isotope over another. This results in end products with isotopic ratios that differ from the starting material, or fractionation. Some of these processes are temperature dependent; typically, biological fractionation is not. The mixing of two isotopically distinct reservoirs will also change the ratio of isotopes. How their distributions in different types of archaeological materials are used to interpret diet, mobility patterns, climate, and provenience is described in Chapter 13.