Conventional CMOS based IC implementations are soon expected to attain a state of saturation mainly due to technological barriers and physical constraints. A number of nanoelectronic devices that may be viable alternatives to standard CMOS have been put forward, and these could pave way for even lower power circuits. Some worthy contenders are carbon nanotube field effect transistor, nanowire field effect transistor, tunnel field effect transistor, and the single electron transistor. In this chapter, device- and circuit-level techniques have been discussed for designing low power circuits in conventional CMOS. Moreover, various sources of power consumption have been reviewed. It is also attempted to review the major components of power i.e. dynamic, short-circuit, and leakage, followed by basics of device and circuit level techniques to reduce the power components. Further, pertinent features of emerging nanoelectronic devices are discussed. These beyond-CMOS devices are expected to revolutionize the era of electronic systems with their low power & high performance characteristics.