Mercury (Hg) stable isotope geochemistry is a rapid emerging research field. During the past decade, mercury isotope geochemistry has become an essential part of mercury science, and has played an important role for a wide variety of scientific progresses relating to this metal. Recent studies have demonstrated that Hg isotopes can be systematically fractionated during specific environmental processes (e.g., biotic, abiotic and photochemical). Mercury can have both mass-dependent fractionation (MDF) and mass-independent fractionation (MIF). Each fractionation process imparts a diagnostic pattern of isotopic variations on the earth's geochemical reservoirs, and thus Hg isotope ratios can be used to unravel the sources and complex biogeochemical pathways for Hg. Large variations (∼10‰) of both MDF and MIF signatures have been detected in different environmental compartments. The observation of MIF in natural samples suggests that photochemical reaction of Hg species plays an important role in the global cycle of Hg. This paper reviews the analytical methods, the contrasting isotope fractionation during Hg geochemical transformation, the isotopic signatures of different geochemical reservoirs, and the potential uses of Hg isotopes in the future.