  The discovery ten years ago that the expansion of the Universe is
accelerating put in place the present cosmological model, in which the Universe
is composed of 4% baryons, 20% dark matter, and 76% dark energy. Yet the
underlying cause of cosmic acceleration remains a mystery: it 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. In these lectures, I present the observational
evidence for cosmic acceleration and what it has revealed about dark energy,
discuss a few of the theoretical ideas that have been proposed to explain
acceleration, and describe the key observational probes that we hope will shed
light on this enigma in the coming years.
