The document discusses optimizing HPC applications for energy efficiency. It finds that optimizing for speed often optimizes energy use as well, but frequency scaling can save energy for some memory-bound workloads. Tools like Allinea MAP and Performance Reports can measure energy at different levels and help developers and users optimize applications when it makes sense to prioritize energy over time. Measuring whole system energy is important, and opportunities to improve efficiency must consider the opportunity cost of slowing down science.