Ultracapacitors are reviewed for various electrode materials, including carbon, mixed metal oxides, and conducting polymers. Carbon is the most commonly used material, with commercially available devices having an energy density of 3–5 Wh/kg and a power density of 300–500 W/kg. Future developments using carbon are projected to achieve energy densities of 10 Wh/kg or higher with power densities of 1–2 kW/kg. A key challenge is bonding thin electrodes to a current collector with low contact resistance. Ultracapacitors offer high power density, high efficiency, and long shelf and cycle life compared to batteries. They are particularly suitable for applications requiring high power and short recharge times. However, their lower energy density limits their use to applications with small energy requirements. Ultracapacitors store energy through double-layer and pseudo-capacitance mechanisms. Double-layer capacitors store energy in the interface between the electrode and electrolyte, while pseudo-capacitors involve Faradaic reactions. Hybrid capacitors combine double-layer and pseudo-capacitance materials for enhanced performance. Current ultracapacitor technology is under development in the US, Japan, and Europe, with applications in electric and hybrid vehicles, as well as medical and consumer electronics. Carbon double-layer capacitors, pseudo-capacitive materials, and hybrid capacitors are being developed. Future projections indicate that energy densities could reach 10–15 Wh/kg and power densities up to 1–6 kW/kg. Key design and cost issues include electrode thickness, contact resistance, bonding to current collectors, electrolyte resistivity, cell configuration, packaging, material purity, fabrication quality, and material cost. Ultracapacitors must have low resistance, long cycle and shelf life, and reasonable cost to be competitive with batteries. Ultracapacitors are inherently high power devices, but their energy density is lower than batteries. They are suitable for applications requiring high power and short recharge times. Future developments aim to improve energy density and power capability while maintaining low cost and long life.Ultracapacitors are reviewed for various electrode materials, including carbon, mixed metal oxides, and conducting polymers. Carbon is the most commonly used material, with commercially available devices having an energy density of 3–5 Wh/kg and a power density of 300–500 W/kg. Future developments using carbon are projected to achieve energy densities of 10 Wh/kg or higher with power densities of 1–2 kW/kg. A key challenge is bonding thin electrodes to a current collector with low contact resistance. Ultracapacitors offer high power density, high efficiency, and long shelf and cycle life compared to batteries. They are particularly suitable for applications requiring high power and short recharge times. However, their lower energy density limits their use to applications with small energy requirements. Ultracapacitors store energy through double-layer and pseudo-capacitance mechanisms. Double-layer capacitors store energy in the interface between the electrode and electrolyte, while pseudo-capacitors involve Faradaic reactions. Hybrid capacitors combine double-layer and pseudo-capacitance materials for enhanced performance. Current ultracapacitor technology is under development in the US, Japan, and Europe, with applications in electric and hybrid vehicles, as well as medical and consumer electronics. Carbon double-layer capacitors, pseudo-capacitive materials, and hybrid capacitors are being developed. Future projections indicate that energy densities could reach 10–15 Wh/kg and power densities up to 1–6 kW/kg. Key design and cost issues include electrode thickness, contact resistance, bonding to current collectors, electrolyte resistivity, cell configuration, packaging, material purity, fabrication quality, and material cost. Ultracapacitors must have low resistance, long cycle and shelf life, and reasonable cost to be competitive with batteries. Ultracapacitors are inherently high power devices, but their energy density is lower than batteries. They are suitable for applications requiring high power and short recharge times. Future developments aim to improve energy density and power capability while maintaining low cost and long life.