The book describes methods of modeling, planning and implementing electric energy storage systems. Energy storage becomes an important issue when more and more electric power is generated by wind mills and photovoltaics systems, because green energy is more volatile. So energy storage is necessary to guarantee safe and secure electric energy supply.Market and power system oriented operations of electric energy storage require different planning methods and different algorithms for searching the optimal solution. These methods are described in detail for energy storage implementations in generation, transmission and distribution levels. Economic aspects are considered.For many years, the authors have been developing smart grid solutions as well as a methology of modeling and planning electric energy storage usage. The aim has been to increase the flexibility of the power system heading for an energy system which is completely generated by green energy.