Abstract: Next generation gravitational wave detectors have a chance of verifying the existence of a cosmological first order transition. To make quantitative statements requires a proper understanding of the process of taking a model with a given set of parameters, converting those parameters into macroscopic thermal parameters and converting those thermal parameters into observables. In this talk I will quantify the theoretical uncertainties arising from scale dependence, the inhomogeneous nature of the back ground, gauge dependence as well as converting calculations done in a Minkowski background to an expanding Universe. The relative changes are significant, ranging from a factor of a few to multiple order of magnitude in the peak amplitude. I also show how to perform more rigorous treatments of these theoretical uncertainties.