Processing information (analysis, storing, retrieving, sharing) is the primary function of modern computer-based information systems. Systems of this type generally require an input flow of exergy (available energy) to function. Information processing systems now are evolving in two directions. One direction is toward bigger and more sophisticated systems. The other is toward systems that are more compact and portable. In both cases, the energy efficiency is becoming an increasingly important design issue. This paper summarizes an exploration of the exergy cost of processing information at the component and system levels in state-of-the-art information processing systems. The energy efficiency characteristics of computer-based information technologies are also compared to estimates of the energy efficiency of biological information processing in brains of mammals. Energy efficiencies of processors and systems are quantified in terms of the ratio of processing capacity to the exergy input rate. Available data suggest that for recent generations of processors, the ratio of processing capacity to exergy input rate has been increasing proportional to the square root of processor speed. Despite this increase, the energy efficiency of computer-based systems is currently substantially below the estimated efficiency of biological systems. Unless processor energy efficiencies are greatly increased, the development of information processing systems that match human brain performance will be hindered by the need for large power supplies and high-capacity heat rejection systems.