The integration of Energy Storage Systems (ESSs) in Micro Grids (MGs) is becoming mandatory for the optimal management of the energy flows and to make the MG an efficient and self-sustainable system, further able to provide ancillary services to the main network. The present paper includes an overview on the services and benefits that an ESS, based on batteries technology, can provide in a MG, considering the classification of the recent DOE/EPRI Electricity Storage Handbook and the recent updates of the Italian Standards dealing with storage. Then the paper shows some experimental results achieved in a real MG thanks to the integration of an ESS, Lithium-Polymer batteries based. The system is a prototype available at ENEA (Italian National Agency for New Technologies, Energy and Sustainable Economic Development) labs in Rome. The authors describe it in details, including its control functions, and give the results of some tests performed at ENEA labs.