Globalization has become an unstoppable trend in nowadays world economy. It brings both risks and opportunities to a country. In order to seize the best part of globalization and avoid possible harms, it is important to comprehend the mechanisms of how openness impacts the overall economy. My dissertation includes three essays that contribute to understanding the impact of openness for the macro-economy. My first chapter starts from two novel observations - 1. Bilateral migration is pervasive across OECD countries, both for high-skilled and low-skilled workers; 2. Foreign affiliates of multinational corporations (MNCs) tend to hire a significantly larger fraction of migrant workers than domestic firms. These two observations challenge the traditional migration models, which assume foreign and native workers within a skill group are homogeneous. These facts also indicate that there exists a tight connection between migration and multinational corporations' activities. I formalize these two points into a general equilibrium model and demonstrate how MNCs and migration can come together to explain the aforementioned observations, and their welfare implications. In my second chapter, I use the theoretical model I developed in Chapter One for a quantitative discussion of immigration policies between the U.S. and Canada. I calibrate the model and implement counterfactual experiments to address two general policy considerations - the effect of migration quotas, and the welfare implication of moving cost adjustments. Contrary to common belief, I find that migration quotas have negative effect on native workers' real income. Further, lower moving costs in general help improving the welfare of workers from both countries. Finally, in the third chapter, I provide a theory to explain the observation that, before WWII, openness is general negatively related to long-term growth, while the relationship becomes positive after WWII. I argue that the effectiveness of technology diffusion between two trading economies is the key determinant of the net effect of openness to long-term growth. The more effective is technology diffusion between two countries, the more likely openness is good for their long-term growth.