Over the past decade, we have been building less and less efficient supercomputers, resulting in the construction of substantially larger machine rooms and even new buildings. In addition, because of the thermal power envelope of these supercomputers, a small fortune must be spent to cool them. These infrastructure costs coupled with the additional costs of administering and maintaining such (unreliable) supercomputers dramatically increases their total cost of ownership. As a result, there has been substantial interest in recent years to produce more reliable and more efficient supercomputers that are easy to maintain and use. But how does one quantify efficient supercomputing? That is, what metric should be used to evaluate how efficiently a supercomputer delivers answers?We argue that existing efficiency metrics such as the performance-power ratio are insufficient and motivate the need for a new type of efficiency metric, one that incorporates notions of reliability, availability, productivity, and total cost of ownership (TCO), for instance. In doing so, however, this paper raises more questions than it answers with respect to efficiency. And in the end, we still return to the performance-power ratio as an efficiency metric with respect to power and use it to evaluate a menagerie of processor platforms in order to provide a set of reference data points for the high-performance computing community.