  The gravitational field equations on cosmological scales are obtained by
averaging the Einstein field equations of general relativity. By assuming
spatial homogeneity and isotropy on the largest scales, the local
inhomogeneities affect the dynamics though correction (backreaction) terms,
which can lead to behaviour qualitatively and quantitatively different from the
Friedmann-Lema\^{i}tre-Robertson-Walker models. The effects of averaging on
cosmological observations are discussed. It is argued that, based on estimates
from observational data, the backreaction (and, in particular, the averaged
spatial curvature) can have a very significant dynamical effect on the
evolution of the Universe and must be taken into account in observational
cosmology.
