This paper discusses the development and comparison of two emissions modeling methods for predicting NOx and CO emissions from aircraft gas turbine combustors. We compare an empirical and a physics-based approach. The objective is to assess the strengths and weaknesses of the methods for predicting the emissions of current and potential future gas turbine engines for the purpose of assessing design tradeoffs and interdependencies in a policy-making setting. The empirical method is based on a P3-T3 approach using polynomial fits to certification data. The physics-based method is developed using high-level combustor design parameters and ideal reactors. The predictive capability of each method is assessed by comparing model estimates of NOx and CO emissions to certification data from three different industry combustors.