This paper focuses on ultracapacitors (electrochemical capacitors) as energy storage in vehicle applications and thus evaluates the present state-of-the-art of ultracapacitor technologies and their suitability for use in electric and hybrid drivelines of various types of vehicles. A key consideration in determining the applicability of ultracapacitors for a particular vehicle application is the proper assessment of the energy storage and power requirements. For hybrid-electric vehicles, the key issues are the useable energy requirement and the maximum pulse power at high efficiency. For a Prius size vehicle, if the useable energy storage is about 125 Wh and needed efficiency is 90-95%, analysis shown in this paper indicate that vehicles can be designed using carbon ultracapacitors (both carbon/carbon and hybrid carbon) that yield high fuel economy improvements for all driving cycles and the cost of the ultracapacitors can be competitive with lithium-ion batteries for high volume production and carbon prices of less than $20 kg ร1 . The use of carbon/carbon devices in micro-hybrids is particularly attractive for a control strategy (sawtooth) that permits engine operation near its maximum efficiency using only a 6 kW electric motor. Vehicle projects in transit buses and passenger cars have shown that ultracapacitors have functioned as expected and significant fuel economy improvements have been achieved that are higher than would have been possible using batteries because of the higher round-trip efficiencies of the ultracapacitors. Ultracapacitors have particular advantages for use in fuel cell powered vehicles in which it is likely they can be used without interface electronics. Development of hybrid carbon devices is continuing showing energy densities of 12 Wh kg ร1 and a high efficiency power density of about 1000 W kg ร1 . Vehicle simulations using those devices have shown that increased power capability in such devices is needed before full advantage can be taken of their increased energy density compared with carbon/carbon devices in some vehicle applications. Energy storage system considerations indicate that combinations of ultracapacitors and advanced batteries (Wh kg ร1 4200) are likely to prove advantageous in the future as such batteries are developed. This is likely to be the case in plug-in hybrids with high-power electric motors for which it may be difficult to limit the size and weight of the energy storage unit even using advanced batteries.