If current technology scaling trends hold, leakage power dissipation will soon become the dominant source of power consumption. Caches, due to the fact that they account for the largest fraction of on-chip transistors in most modern processors, are a primary candidate for attacking the leakage problem. While there has been a flurry of research in this area over the last several years, a major question remains unanswered. What is the total potential of existing architectural and circuit techniques to address this important design concern? In this paper, we explore the limits in which existing circuit and architecture technologies may address this growing problem. We find that by using perfect knowledge of the address trace to carefully apply sleep and drowsy modes, the total leakage power from the instruction cache may be reduced to mere 3.6% of the unoptimized case, and the total from the data cache reduced to only 0.9%. We also present a complete parameterized model to determine the optimal leakage savings while the implementation technology changes over time. We futher suggest how such limits might be approached using a form of prefetching for low power.