  The acceleration of the cosmic expansion is a fundamental challenge to
standard models of particle physics and cosmology. The new physics of dark
energy may lie in the nature of gravity, the quantum vacuum, or extra
dimensions. I give a brief overview of the puzzles and possibilities of dark
energy, and discuss the confrontation of a wide variety of "beyond Einstein"
models with the latest data, showing what we currently know and what we must
seek to learn. Next generation experiments using a variety of cosmological
probes will deeply explore dark energy, dark matter, and gravitation.
