  One possible explanation for the present observed acceleration of the
Universe is the breakdown of homogeneity and isotropy due to the formation of
non-linear structures. How inhomogeneities affect the averaged cosmological
expansion rate and lead to late-time acceleration is generally considered to be
due to some backreaction mechanism. General Relativity together with
pressure-free matter have until recently been considered as the sole
ingredients for averaged calculations. In this communication we focus our
attention on more general scenarios, including imperfect fluids as well as
alternative theories of gravity, and apply an averaging procedure to them in
order to determine possible backreaction effects. For illustrative purposes, we
present our results for dark energy models, quintessence and Brans-Dicke
theories. We also provide a discussion about the limitations of frame choices
in the averaging procedure.

Cosmology and Nongalactic Astrophysics;General Relativity and Quantum Cosmology