For many years after its appearance, general relativity (GR) was regarded as an exotic extension of Newtonian gravity, that was only necessary for highprecision measurements in the Solar System and for describing the expansion of the Universe. However, the increasing precision of physical and astronomical measurement is transforming GR into an indispensable tool, and not merely a small correction to Newton's theory.

It is commonly stated that we have entered the era of precision cosmology, in which a number of important observations have reached a degree of precision, and a level of agreement with theory, that is comparable with many Earth-based physics experiments. One of the consequences of this advance is the need to examine at what point our usual, well-worn assumptions begin to compromise the accuracy of our models, and whether more general theoretical methods are needed to maintain calculational accuracy. Historically, each advance in astronomical measurement has produced many new discoveries, and revealed more of the structure of the cosmos, such as voids, walls, filaments, etc. As we map out the Universe around us – its mass distribution and flow patterns – in ever greater detail, the nonlinear behaviour of cosmic structures will become increasingly apparent, and the methods of inhomogeneous cosmology will come into their own. Inhomogeneous solutions of Einstein's field equations provide models of both small and large structures that are fully nonlinear.

It is widely assumed that the Universe, when viewed on a large enough scale, is homogeneous and can be described by an FLRW model. The successes of the Concordance model are built on using a spatially homogeneous and isotropic background metric combined with first-order perturbation theory.