The energy usage of computer systems is becom- ing more important, especially for battery operated systems. Displays, disks, and cpus, in that order, use the most energy. Reducing the energy used by displays and disks has been studied elsewhere; this paper con- siders a new method for reducing the energy used by the cpu. We introduce a new metric for cpu energy per- formance, millions-of-instructions-per-joule (MIPJ). We examine a class of methods to reduce MIPJ that are characterized by dynamic control of system clock speed by the operating system scheduler. Reducing clock speed alone does not reduce MIPJ, since to do the same work the system must run longer. However, a number of methods are available for reducing energy with reduced clock-speed, such as reducing the voltage [Chandrakasan et al 1992] [Horowitz 1993] or using reversible [Younis and Knight 1993] or adiabatic logic [Athas et al 1994]. What are the right scheduling algorithms for tak- ing advantage of reduced clock-speed, especially in the presence of applications demanding ever more instruc- tions-per-second? We consider several methods for varying the clock speed dynamically under control of the operating system, and examine the performance of these methods against workstation traces. The primary result is that by adjusting the clock speed at a fine grain, substantial CPU energy can be saved with a lim- ited impact on performance.