Although Petaflop systems have only recently become a reality, scientists and governments are eagerly anticipating Exascale capabilities. There is a significant effort to achieve an Exaflop within the decade. However, unlike Petascale systems, Exascale systems will not be straightforward extrapolations from predecessors, and success is not a foregone conclusion. The chief combatant is power consumption: systems must become enormously more energy-efficient to make Exascale practical. General-purpose cores are too power-hungry, and thus more specialized forms of parallelism must be exploited. However, it is highly likely that this will require changes in applications and programming models, especially considering that these systems will require tens of millions of cores. Complicating matters further, error detection and resiliency are already major issues for Petascale systems, and improving fault tolerance typically requires more energy. We will discuss innovations in technology, architecture, and software tha