Why is the universe accelerating

The remarkable discovery of cosmic acceleration poses two fundamental questions. (1) Does acceleration reflect the presence of a new energy component or the breakdown of General Relativity on cosmological scales? (2) If acceleration is caused by a new energy component, is it constant in space and time as expected for fundamental vacuum energy, or does it show evolution or variation that imply a dynamical field? After briefly reviewing some of the theoretical ideas for explaining cosmic acceleration, I will turn to observational methods for addressing these questions, with emphasis on recent results from baryon acoustic oscillations (BAO) measured in the Sloan Digital Sky Survey and plans for future facilities such as the Dark Energy Survey, BigBOSS, LSST, Euclid, and WFIRST.