Controlling the magnetic properties of solids by electric fields has been an interesting research subject, not only because of the intriguing correlation between the electric and magnetic orders in solid-state systems, but also the potential applications in ultra-low energy spintronic devices. In the past, research has mostly been carried out with multiferroic materials and magnetic semiconductors. Recently, more effort was focused on 3d transition ferromagnetic metals. Especially in heavy metal/ferromagnet/oxide (HM/FM/oxide) structures where the magnetic anisotropy has an interfacial origin, electric fields can cause a marked change in the magnetic anisotropy energy. This voltage-controlled magnetic anisotropy (VCMA) can be understood by the electric field induced charge transfer among different d orbitals of the FM. The order of this effect is around 100 fJ/Vm and it vanishes with the removing of the electric fields.