Energy storage systems (ESS) have become increasingly vital for ensuring the stability and reliability of power grids, particularly with the growing share of renewable energy sources like wind and solar. Given that renewable energy generation is inherently intermittent and weather-dependent, ESS provide the crucial ability to store excess energy during periods of high production and release it during times of high demand or low generation. This capability effectively decouples energy generation from consumption, stabilizing grid fluctuations and ensuring a continuous and reliable power supply. As a result, ESS have gained significant attention over the past decade, as they play a key role in supporting the transition toward more sustainable and resilient energy systems. This is reflected in their prominence within activities of CIGRE SC C6. A wide range of energy storage technologies has developed over time, each varying considerably in performance and technological maturity. These technologies include direct energy storage, which retains surplus energy in electrical form, and indirect storage, where excess energy is converted into alternative forms such as thermal or chemical energy. While each technology offers unique advantages, energy losses incurred during charging, discharging, and storage processes can reduce overall efficiency. This chapter offers a comprehensive analysis, evaluation, and classification of the most common energy storage technologies, emphasizing their strengths, challenges, and suitability for different applications. It covers a diverse spectrum of storage types, including electrical, electrochemical, chemical, thermal, and mechanical systems. The evolution of ESS technologies is explored, from early-stage developments to advanced systems in use today, such as lithium-ion batteries, pumped hydro storage (PHS), and hydrogen storage (H2). Emerging technologies, including latent and thermochemical storage, are also examined for their potential to address long-term energy storage needs. Looking ahead, continued advancements in ESS, particularly in electrochemical and power-to-gas (PtG) systems, promise enhanced efficiency, longer storage durations, and higher energy densities. As these technologies mature and their costs decrease, they will become more widely integrated into residential and industrial energy systems, further facilitating the renewable energy transition. The integration of ESS across multiple sectors—such as electricity, heating, and transportation—holds the potential to create a more sustainable, greener energy infrastructure. By supporting the widespread adoption of renewable energy sources, ESS are key to addressing the challenges of grid stability, energy security, and sustainability in a decarbonized world. Ultimately, ESS will remain a cornerstone of future energy systems, particularly in active distribution networks, enabling the full integration of renewables and helping meet the pressing needs of a modern, low-carbon energy landscape.