As new geological tracers, Mg isotopes attract more and more attentions from international geologists. Great achievements have been made in the Mg isotope geochemistry. Recent studies have been focusing on two aspects. First, Mg isotopic compositions of the major terrestrial reservoirs and chondrites have been investigated. Chondrites and the mantle have homogeneous and similar Mg isotopic compositions, with the average δ26Mg values of -0.28±0.06‰ and -0.25±0.07‰, respectively. By contrast, the upper continental crust and hydrosphere are highly heterogeneous, with δ26Mg values varying from -4.84‰ to +0.92‰ and from -2.93‰ to +1.13‰, respectively. Second, studies on behaviors of Mg isotopes during geological and physicochemical processes suggest that: (1) large Mg isotope fractionation occurs during continental weathering with heavy Mg isotopes retained in the weathered products and light Mg isotopes released into the hydrosphere; (2) equilibrium Mg isotope fractionation during magma differentiation is limited; (3) significant kinetic Mg isotope fractionation occurs during chemical and thermal diffusion at high temperatures. Based on these studies, Mg isotopes have been used to trace accretion of the Earth and recycling of crustal materials, and, in the near future, may potentially be applied to tracing chemical evolution of the continental crust and be used as geological thermometers.