In current servers, the lowest energy-efficiency region corresponds to their most common operating mode. Addressing this perfect mismatch will require significant rethinking of components and systems. To that end, we propose that energy proportionality should become a primary design goal. Energy-proportional designs would enable large energy savings in servers, potentially doubling their efficiency in real-life use. Achieving energy proportionality will require significant improvements in the energy usage profile of every system component, particularly the memory and disk subsystems. Although our experience in the server space motivates these observations, we believe that energy-proportional computing also will benefit other types of computing devices.