Electrochemical capacitors (ECs) are essential for storing and delivering electrical energy quickly, but their energy density needs to be increased to power flexible and wearable electronics and larger equipment. This review summarizes recent progress in materials for ECs over the past decade and outlines key future research directions. It discusses electrical double-layer capacitors (EDLCs) based on high-surface-area carbons, pseudocapacitive materials like oxides and MXenes, and emerging microdevices for the Internet of Things. The review highlights the need for new nanostructured electrode materials and matching electrolytes to maximize energy storage and delivery speed, as well as different manufacturing methods for future electronic devices. Scientifically justified metrics for testing, comparing, and optimizing various ECs are provided. ECs are important for powering hybrid/electric vehicles and increasing the number of electronic gadgets, as well as storing energy from intermittent sources like wind and sun. High-performance energy-storage systems are needed in our daily lives, as we want our electronic devices to hold their charge and operate throughout the day. ECs play an increasing role in satisfying the demand for high-rate energy harvesting, storage, and delivery. Since then, the need for versatile, ubiquitous, high-power, and high-energy-density storage has only increased. The Ragone plot in Figure 1 shows the power versus energy performance of various energy-storage systems. Batteries are in the high-energy and low-power region, while ECs have higher power but lower energy density. ECs store charge via fast, surface-confined processes, resulting in long cycle life and fast charge rates. These features make ECs useful in applications ranging from small devices for power electronics to large cells for automotive transportation. ECs are also used for grid energy storage, power quality, and frequency regulation. Ragone plots are widely used to provide information on power versus energy density of devices. The colored areas in Figure 1 correspond to performance obtained using the same charge and discharge current, while areas enclosed by dashed lines refer only to discharge performance. The plot highlights the power capability of ECs compared with batteries. However, such a plot gives only part of the information. Although Li-ion batteries can be discharged in a few tens of seconds, their energy efficiency and cycle life will be greatly affected, making ECs preferred for high-rate applications when a long cycle life is required. The review summarizes progress in ECs over the past decade and shows key perspectives for future materials research, covering porous carbons, pseudocapacitive materials, and emerging fields like wearable energy storage and microdevices for the Internet of Things. It discusses EDLCs, which store charge electrostatically through reversible adsorption of electrolyte ions onto high-surface-area carbon materials. EDLCs have high power but limited energy density, and their challenge is to increase the amount of energy stored. Pseudocapacitive materials, such as oxides and MXenes, offer higher energy density than EDLCElectrochemical capacitors (ECs) are essential for storing and delivering electrical energy quickly, but their energy density needs to be increased to power flexible and wearable electronics and larger equipment. This review summarizes recent progress in materials for ECs over the past decade and outlines key future research directions. It discusses electrical double-layer capacitors (EDLCs) based on high-surface-area carbons, pseudocapacitive materials like oxides and MXenes, and emerging microdevices for the Internet of Things. The review highlights the need for new nanostructured electrode materials and matching electrolytes to maximize energy storage and delivery speed, as well as different manufacturing methods for future electronic devices. Scientifically justified metrics for testing, comparing, and optimizing various ECs are provided. ECs are important for powering hybrid/electric vehicles and increasing the number of electronic gadgets, as well as storing energy from intermittent sources like wind and sun. High-performance energy-storage systems are needed in our daily lives, as we want our electronic devices to hold their charge and operate throughout the day. ECs play an increasing role in satisfying the demand for high-rate energy harvesting, storage, and delivery. Since then, the need for versatile, ubiquitous, high-power, and high-energy-density storage has only increased. The Ragone plot in Figure 1 shows the power versus energy performance of various energy-storage systems. Batteries are in the high-energy and low-power region, while ECs have higher power but lower energy density. ECs store charge via fast, surface-confined processes, resulting in long cycle life and fast charge rates. These features make ECs useful in applications ranging from small devices for power electronics to large cells for automotive transportation. ECs are also used for grid energy storage, power quality, and frequency regulation. Ragone plots are widely used to provide information on power versus energy density of devices. The colored areas in Figure 1 correspond to performance obtained using the same charge and discharge current, while areas enclosed by dashed lines refer only to discharge performance. The plot highlights the power capability of ECs compared with batteries. However, such a plot gives only part of the information. Although Li-ion batteries can be discharged in a few tens of seconds, their energy efficiency and cycle life will be greatly affected, making ECs preferred for high-rate applications when a long cycle life is required. The review summarizes progress in ECs over the past decade and shows key perspectives for future materials research, covering porous carbons, pseudocapacitive materials, and emerging fields like wearable energy storage and microdevices for the Internet of Things. It discusses EDLCs, which store charge electrostatically through reversible adsorption of electrolyte ions onto high-surface-area carbon materials. EDLCs have high power but limited energy density, and their challenge is to increase the amount of energy stored. Pseudocapacitive materials, such as oxides and MXenes, offer higher energy density than EDLC