The main provisions of the theory of REE fractionation in natural processes are considered. For the first time, geochemical REE cycles are described in magmatic and sedimentary processes, including heterogeneity of REE distribution in the mantle, the behavior of REEs in the processes of smelting primary magmas and their fractional crystallization and segregation, as well as the separation of REEs in the formation of weathering crust, transfer of REEs by natural waters and during continental and oceanic sedimentation in different tectonic conditions.