  This article looks at how inhomogeneous spacetime models may be significant
for cosmology. First it looks at how the averaging process may affect large
scale dynamics, with backreaction effects leading to effective contributions to
the averaged energy-momentum tensor. Secondly it considers how local
inhomogeneities may affect cosmological observations in cosmology, possibly
significantly affecting the concordance model parameters. Thirdly it presents
the possibility that the universe is spatially inhomogeneous on Hubble scales,
with a violation of the Copernican principle leading to an apparent
acceleration of the universe. This could perhaps even remove the need for the
postulate of dark energy.
