We build a dynamic general equilibrium model with 2 countries, horizontal and vertical multinational activity and endogenous domestic and foreign investment. It is found that horizontal multinational activity always leads to a complementary relationship between domestic and foreign investment. Vertical multinational activity, in contrast, leads to either a substitutional or complementary relationship between domestic and foreign investment, depending on the firms' technologies. We test the theoretical implications with a panel of U.S. multinationals and find empirical support.