Fostering the green energy world has become a hot topic in recent years. More and more hybrid vehicles find their way into the consumer market. Batteries or ultracapacitors are usually employed as the second energy source in hybrid vehicles. Compared with batteries, ultracapacitors have more advantages such as higher energy densities, long life cycles and environment-friendly. In order to have a good control strategy in hybrid vehicle to take advantage of ultracapacitor, its model must be accurate. In this book, short-term and long-term models have been developed. For the short-term models, high order transfer functions consistent with an RC ladder model are assumed. Transfer function coefficients are identified by a least squares algorithm based on experimental data consisting of time-varying current excitations and the resulting terminal voltage responses. A long-term model with six RC branches is developed by fitting the terminal voltage transient response to an impulse charging current. From the ultracapacitor models derived, terminal voltages under different current profiles can be determined accurately over the time frame of one hour or two months.