Low power and energy consumption will always be an essential requirement in many real-time embedded applications. Voltage scaling is a relative novel approach to reducing energy consumption. The idea is that a processor can be run either at high or at low voltage: at high voltage, the clock rate is high but so is the power consumption; at low voltage the clock rate is lower, but the power consumption drops by a greater factor. This immediately suggests a powerful approach to lowering energy consumption in real-time systems. In this paper, we introduce static and dynamic algorithms to control processor voltage to reduce energy consumption. We demonstrate by simulation that our algorithms can significantly reduce total energy consumption.