[영문]This paper is dedicated to Sherwin Rosen?for all his encouragement. We are indebted to Daron Acemoglu, George Bound, Alan Deardorff, Gilles Duranton, Gordon Hanson, Elhanan Helpman, Jim Levinsohn, Diego Puga, Aloysius Siow, Nadia Soboleva, and Alwyn Young for constructive comments. Two anonymous referees and the editor (Anil Kashyap) provided excellent suggestions that transformed the paper. Derek Neal was kind enough to pull a confused Trefler aside after an Applications Workshop and initiate him into the wonderful world of Roy. We are particularly grateful to Jim Heckman and Sherwin Rosen for their guidance and patience. Funding for this project has been generously provided by the Social Sciences and Humanities Research Council of Canada.Each worker brings a bundle of skills to the workplace, for example, quantitative and communication skills. Since employers must take this bundle as a package deal, they choose workers with just the right mix of skills. We show that international differences in the distribution of worker skill bundles?for example, Japan's abundance of workers with a modest mix of both quantitative and teamwork skills?have important implications for international trade, industrial structure, and domestic income distribution. Formally, we model two-dimensional worker heterogeneity and show that the second moments of the distribution of skills are critical, as in the Roy model.