Book excerpt: Almost all of the large literature on international trade with imperfect competition assumes exogenous market structures. The purpose of this paper is to develop a simple model that generates alternative market structures as Nash equilibria for different parameterizations of the basic model. Familiar configurations such as a duopoly competing in exports or a single multinational producing in both markets arise as special cases. Small tax-policy changes can produce large welfare effects as the equilibrium market structure shifts, implying discontinuous jumps in prices, quantities, and profits.