  The post-Newtonian (PN) perturbative framework has been successful in
understanding the slow-motion, weak field limit of Einstein's theory of gravity
on solar system scales, and for isolated astrophysical systems. The
parameterized post-Newtonian (PPN) formalism extended the PN framework to put
very tight constraints on deviations from Einstein's theory on the
aforementioned scales and systems. In this work, we extended and applied the
post-Newtonian formalism to cosmological scales. We first used it to construct
a cosmological model to understand the effect of regularly arranged point
sources on the background expansion. Here we found that at higher orders we
obtained a small radiation-like correction to the standard
Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) equations, for a
matter-dominated universe. This radiation-like correction was purely due to the
inhomogeneity of our model, and the non-linearity of Einstein's field
equations. We also extended the post-Newtonian formalism to include other forms
of matter that are cosmologically relevant, such as radiation and a
cosmological constant, and studied the non-linear effects they might have on
the background expansion. Then we constructed an extension of the parameterized
post-Newtonian formalism (PPN) to cosmological scales. We used it to
parameterize the background expansion of the universe as well as first-order
perturbations in cosmology, using four functions of time. We gave examples of
how our parameterization would work for dark energy models and scalar-tensor
and vector-tensor theories of gravity. In the final part of this work, we
studied how light propagation behaves in an inhomogeneous post-Newtonian
cosmology with matter and a cosmological constant. We used it to understand the
effect that inhomogeneities would have on observables.
