  The discovery ten years ago that the expansion of the Universe is
accelerating put in place the last major building block of the present
cosmological model, in which the Universe is composed of 4% baryons, 20% dark
matter, and 76% dark energy. At the same time, it posed one of the most
profound mysteries in all of science, with deep connections to both
astrophysics and particle physics. Cosmic acceleration could arise from the
repulsive gravity of dark energy -- for example, the quantum energy of the
vacuum -- or it may signal that General Relativity breaks down on cosmological
scales and must be replaced. We review the present observational evidence for
cosmic acceleration and what it has revealed about dark energy, discuss the
various theoretical ideas that have been proposed to explain acceleration, and
describe the key observational probes that will shed light on this enigma in
the coming years.
