This thesis comprises of three empirical essays. The first essay studies the effect of NAFTA on US-Mexico productivity evolution. Introducing refinements on the previous literature in several fronts, it finds no clear evidence for NAFTA's contribution to the TFP convergence, which is in a sharp contrast with the previous literature. The second and third essays relate to the Heterogeneous Firms Trade models. Using NAFTA's case, the second essay shows a positive effect of NAFTA on export diversification of Mexico as both models predict. However, the study supports neither model in terms of its prediction on unit prices of export goods. The third essay classifies export goods into price-competition goods and quality-competition goods. It finds EU countries' high proportion of quality-competition goods and China's high proportion of price-competition goods, but leaves some open questions to be further explored