Efficient clean energy storage is developing at a fast pace in the 21st century, aimed at building a fossil fuel-free society. Due to high theoretical efficiency in converting chemical to electrical energy, promise of electrochemical energy storage technologies has triggered numerous efforts in improving energy and power performance metrics. Electrochemical capacitors, also known as supercapacitors, keep evolving into new domains of research and development with focus on fast charging, high energy density, and long-lasting energy storage technologies. Specifically, supercapacitors can be a better choice over batteries in high power density applications with typical charging time scales of a few seconds for low energy density applications (5-10Wh/kg). In this chapter, we dwell upon fundamental aspects and historical developments of electrochemical capacitors. It covers fundamental understanding of charge storage processes, general properties, and transition from electrical double-layer capacitors to high rate-high energy pseudocapacitive energy storage. This chapter also provides progress on state-of-the-art capacitive energy storage devices including hybrid capacitors and on-chip microsupercapacitors. At the end, perspectives on new materials development and new mechanisms for improving energy storage technologies will be discussed. © 2022 Elsevier Inc. All rights reserved.