Energy storage is not a new concept but is currently getting increasing importance in the context of energy transition paradigm. Indeed, it is expected to play a key role as an enabling technology for lowering the carbon footprint of the electric power system. In fact, the growing development of renewable energy resources and their increasing share in the energy mix, are introducing significant challenges to the existing power grid due to the high variability of these sources/loads. In particular, maintaining the generation-consumption balance of the electric power in real time, as well as the overall power system security, when these special energy sources/loads are present at a significant scale is a major concern. With competitive energy storage, it will be possible to introduce more flexibility in the electrical system thus helping it to better manage the overall energy balance with better system response in case of severe contingencies. Energy storage technologies were historically used for managing the load curve while observing generation dynamic constraints. The most well-known storage technology is the pumped hydro storage where the energy is stored in a hydraulic form (water potential energy). With the event of open access and the corresponding unbundling of electric power industry segments, valorizing energy storage options under market conditions has become tricky. The major present barriers for deploying energy storage systems (ESS) are high cost, competitive economic value, efficiency and energy density, together with energy policies. The new energy paradigm has put a new emphasis on energy storage, and many research roadmaps have pointed out the need for overcoming the current barriers. The decision makers' awareness of the importance of energy storage is also on the rise. However, adequate incentives for encouraging massive deployment of ESS and storage technology within the electric power system are still lacking. Currently, most of the effort is dedicated to in situ demonstration projects in striving for smarter grids and support of innovations with the corresponding proofs of concept and feedback experience. Additionally, different grid applications are assessed for both centralized to decentralized uses. Various energy storage applications for frequency regulation, voltage support, investment optimization, or peak shaving are under consideration. In this article, some of the main energy storage technologies will be reviewed according to their main application domains. That will be followed by a focus on battery energy storage. Some key elements of battery management system (BMS) technologies and ESS architecture and characterization will be addressed. Then some aspects of ESS protection will be presented and the key trends and indications of emerging concepts for energy storage will be identified.