  General Relativity (GR) is a phenomenologically successful theory that rests
on firm foundations, but has not been tested on cosmological scales. The advent
of dark energy (and possibly even the requirement of cold dark matter), has
increased the need for testing modifications to GR, as the inference of such
otherwise undetected fluids, depends crucially on the theory of gravity. In
this work I outline a general scheme for constructing consistent and covariant
modifications to the Einstein equations. This framework is such that there is a
clear connection between the modification and the underlying field content that
produces it. I conclude by a simple metric based modification of the
fluctuation equations for which the background is exact Lambda-CDM and present
its impact on observations of the cosmic microwave background radiation.
