Magmatic rocks of mafic composition are widespread in all types of tectonic structures and geodynamic settings, including folded belts, mid-oceanic ridges, oceanic islands, cratons, as well as the moon. Rocks related to this voluminous group have been formed throughout geological history of the earth’s crust until the present time. Parent melts of mafic rocks have been intruded and crystallized as plutonic massifs at abyssal, mesoabyssal, and hypabyssal depths, and as subvolcanic and volcanic formations under subsurface and surface conditions. Significant variations in chemical and mineral compositions of mafic rocks are caused by a wide range of physical and chemical conditions, at which generation, fractionation, contamination, and crystallization of their parent melts took place. The same processes have predetermined the wide variations in rare element contents of rocks, including rare earth contents in mafic rocks.