Phase separation phenomena in metallic glass systems are reviewed. First, a brief introduction to the enhanced glass forming ability and the phase separation in metallic glass systems is given. Nano-scale phase separation observed in frozen-in glass is discussed in marginal glass formers such as Pd-, Fe, Al-base metallic glass systems as well as in bulk glass formers in Cu-, Zr- and Mg-base metallic glass systems. Since retention of glass forming ability is essential in phase separating glasses, thermodynamic and kinetic conditions for enhanced glass forming ability is introduced. Several thermodynamic aspects for decomposition by liquid–liquid phase separation which include stability conditions, decomposition in the multicomponent system, types of miscibility gap, calculation of bimodal and spinodal curves are introduced as a background for design of metallic glasses phase separating in the liquid state. The main mechanisms for phase separation are nucleation and growth mechanism and spinodal decomposition mechanism. The metallic glass systems which include an atom pair with large positive enthalpy of mixing decompose by liquid–liquid phase separation separate in the liquid state, forming two typical types of microstructure: droplet or interconnected type microstructures depending on the mechanism of phase separation. By calculation of the tile line, it is possible to design phase separating metallic glasses with high glass forming ability. Eventually, bulk-type phase separating metallic glasses with mm scale can be synthesized. Moreover, if there is a group of three atoms with large positive enthalpy of mixing in the liquid state in the multicomponent system, occurrence of three phase separation in the liquid state is possible. There are several parameters affecting the microstructure evolution during phase separation, which includes glass transition and critical temperatures, shape of bimodal and spinodal curves and quenching conditions. Phase separation in the solid state of the frozen-in glasses can give a clue on the phase separation prior to crystallization or direct nanocrystallization from the amorphous matrix. The amplitude of the composition fluctuation with a fixed wavelength can grow by heating in the solid state, if the spinodal decomposition. Finally, some advantages of phase separation phenomena in metallic glass systems are highlighted.