In this talk I will describe recent progress modeling cosmological observables in a general relativistic setting. These advances provide us with insight into subtle gravitational effects that allow us to test general relativity in new regimes; yet if unaccounted for, these same effects may mislead us in our search for new physics. I will describe how these models not only allow us to study the behavior of general relativity at a nonlinear level, but provide us with a way to test the fundamental assumptions our cosmological models are built upon.