Energy storage systems can be considered today as new means to adapt the variations of the power demand to the given level of power generation. In the context of the use of Renewable Energies or other distributed power generation, they can be used also as additional sources, or as energy buffers in the case of non-schedulable or stochastic generation, like wind- or photovoltaic power. This paper will describe different means of electrical energy storage, including systems where other energy transformations are needed, from electrical energy to mechanical, involving potential or kinetic forms, as well as chemical transformations. One dominant aspect will be studied, and concerns the conversion of electrical power, from the point of view of power-electronics. When direct electrical energy carriers like magnets, capacitors or batteries are used, DC-to-DC converters are needed as controlled voltage and current sources. More complex converters will be considered like torque controlled drives in the case of electromechanical energy conversion. In this case, variable speed drives play an important role, as in flywheels, but also for modern pumped-hydro power.