As we move to integration levels of 1,000-core processor chips, it is clear that energy and power consumption are the most formidable obstacles. To construct such a chip, we need to rethink the whole compute stack from the ground up for energy efficiency - and attain Extreme-Scale Computing. First of all, we want to operate at low voltage, since this is the point of maximum energy efficiency. Unfortunately, in such an environment, we have to tackle substantial process variation. Hence, it is important to design efficient voltage regulation, so that each region of the chip can operate at the most efficient voltage and frequency point. At the architecture level, we require simple cores organized in a hierarchy of clusters. Moreover, we also need techniques to reduce the leakage of on-chip memories and to lower the voltage guardbands of logic. Finally, data movement should be minimized, through both hardware and software techniques. With a systematic approach that cuts across multiple layers of the computing stack, we can deliver the required energy efficiencies.