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Science and engineering (S&E) workers are the fundamental inputs into scientific innovation and technology adoption. In the United States, more than 20% of the S&E workers are immigrants from developing countries. In this paper, I evaluate the impact of such brain drain from non-OECD (i.e., developing) countries using a multi-country endogenous growth model. The proposed framework introduces and quantifies a "frontier growth effect" of skilled migration: migrants from developing countries create more frontier knowledge in the U.S., and the non-rivalrous knowledge diffuses to all countries. In particular, each source country is able to adopt technology invented by migrants from other countries, a previously ignored externality of skilled migration. I quantify the model by matching both micro and macro moments, and then consider counterfactuals wherein U.S. immigration policy changes. My results suggest that a policy - which doubles the number of immigrants from every non-OECD country - would boost U.S. productivity growth by 0.1 percentage point per year, and improve average welfare in the U.S. by 3.3%. Such a policy can also benefit the source countries because of the "frontier growth effect". Taking India as an example source country, I find that the same policy would lead to faster long-run growth and a 0.9% increase in average welfare in India. This welfare gain in India is largely the result of additional non-Indian migrants, indicating the significance of the previously overlooked externality.