Mercury (Hg) is a redox-active trace metal that is viewed internationally as a priority pollutant. Research into Hg stable isotope biogeochemistry is rapidly providing new insight into the behavior of Hg. With the recent discovery that Hg can exhibit both mass-dependent (MDF) and mass-independent fractionation (MIF) (range of >6‰ for both), Hg isotopes are providing a valuable new tool for tracing this important toxin through the environment. MDF alone, which occurs during redox transformations, biological cycling, and volatilization of Hg, can be exploited to increase understanding of the processes that control Hg distribution and bioaccumulation. The addition of MIF signatures greatly increases the usefulness of Hg isotopes because MIF provides a unique fingerprint of specific chemical pathways, such as photochemical reduction.