Energy Storage Systems theme is a component of Encyclopedia of Energy Sciences, Engineering and Technology Resources which is part of the global Encyclopedia of Life Support Systems (EOLSS), an integrated compendium of twenty one Encyclopedias. The Theme is organized into six different topics which represent the main scientific areas of the theme: The first topic, Rationale of Energy Storage and Supply/Demand Matching is devoted to the discussion of essential concepts and the most important aspects of the optimization, establishment and operation of energy storage systems based on six cases as examples. The succeeding four topics are Storage of Thermal Energy; Mechanical Energy Storage; Storage of Electrical Energy; Storage of Chemical Energy and Nuclear Materials. Each of these consists of a topic chapter emphasizing the general aspects and various subject articles explaining the back ground, theory and practice of a specific type of energy storage of that topic. The last topic is transport of energy with emphasis on hydrogen as future energy carrier. It contains detailed review of other modes of energy transport and discussion of environmental effects. Fundamentals and applications of characteristic methods are presented in these volumes. These two volumes are aimed at the following five major target audiences: University and College Students, Educators, Professional Practitioners, Research Personnel and Policy Analysts, Managers, and Decision Makers and NGOs.