  A central theme in cosmology is the perplexing fact that the Universe is
undergoing an accelerating expansion. The latter, one of the most important and
challenging current problems in cosmology, represents a new imbalance in the
governing gravitational field equations. Several candidates, responsible for
this expansion, have been proposed in the literature, in particular, dark
energy models and modified gravity, amongst others. In this paper, we explore
the possibility that the late-time cosmic acceleration is due to infra-red
modifications of Einstein's theory of General Relativity, and review some of
the modified theories of gravity that address this intriguing and exciting
problem facing modern physics.
