The electrochemical ultracapacitor is an emerging technology that promises to play an important role in meeting the demands of electronic devices and systems both now and in the future. This newly available technology of ultracapacitors is making it easier for engineers to balance their use of both energy and power. Energy storage devices like ultracapacitors are normally used along with batteries to compensate for the limited battery power capability. Evidently, the proper control of the energy storage systems presents both a challenge and opportunity for the power and energy management system. This paper traces the history of the development of the technology and explores the principles and theory of operation of the ultracapacitors. The use of ultracapacitors in various applications are discussed and their advantages over alternative technologies are considered. To provide examples with which to outline practical implementation issues, systems incorporating ultracapacitors as vital components are also explored. This paper has aimed to provide a brief overview of ultracapacitor technology as it stands today. Previous development efforts have been described to place the current state of the technology within an historical context. Scientific background has also been covered in order to better understand performance characteristics. Possible applications of ultracapacitor technology have also been described to illustrate the wide range of possibilities that exist. Because of the advantages of charging efficiency, long lifetime, fast response, and wide operating temperature range, it is tempting to try and apply ultracapacitors to any application that requires energy storage. The limitations of the current technology must be fully appreciated, however, and it is important to realize that ultracapacitors are only useful within a finite range of energy and power requirements. Outside of these boundaries other alternatives are likely to be the better solution. The most important thing to remember about ultracapacitors technology is that it is a new and different technology in its own right. There may exist some similarities between ultracapacitor operation and the operation of electrostatic capacitors, but there are fundamental differences that result from the different physical processes involved and these must be appreciated. Problems may be encountered if systems are designed based on the assumption that ultracapacitors behave like normal capacitors. Ultracapacitors are, at any rate, a part of the new wave of advanced energy storage devices that will further the push towards greater energy efficiency and more sustainable alternatives. They will be a useful tool with which to engineer highly efficient electrical and electronic systems, and as the state of the technology advances they will become progressively more commonplace.