This chapter examines the ESSs and their importance in energy systems and especially in renewable energy systems (RESs). It establishes the foundation by giving the essential elements that make energy storage systems (ESSs) necessary for supporting intermittent renewable energy sources. Acting as a fundamental reference, this chapter is particularly valuable for researchers aiming to fully understand the essential role that storage plays in RESs. The importance of ESSs in RESs is discussed at length, with important elements identified.