PA2 has published a booklet on Energy Storage, available under the studies of the PA2 website. The brochure highlights the needs and drivers for energy storage: a cost-efficient energy transition towards a highly energy-efficient and renewable-based energy system for a climate-neutral economy requires a well-developed and smart energy grid, advanced storage and flexibility technologies, backup generation and demand response in order to secure a constant, affordable and sustainable power supply. The document also provides a brief description of the main energy storage technologies and applications at various time scales, ranging from seconds to months, as well as storage sizes, ranging from tens of kWh to tens of GWh, and highlights the substantial role of subsurface energy storage as a large-scale solution.