Approaches to understanding cosmic acceleration

Theoretical approaches to explaining the observed acceleration of the universe are reviewed. We briefly discuss the evidence for cosmic acceleration, and the implications for standard general relativity coupled to conventional sources of energy-momentum. We then address three broad methods of addressing an accelerating universe: the introduction of a cosmological constant, its problems and origins; the possibility of dark energy and the associated challenges for fundamental physics and the option that an infrared modification of general relativity may be responsible for the large-scale behavior of the universe.