Where approved, producers have adopted genetically modified (GM) crops extensively. Yet, areas not adopting GM crops account for large shares of production and consumption. GM crops differ from previous agricultural innovations because consumers may perceive them as fundamentally different from (and potentially inferior to) conventionally grown crops. Many countries maintain restrictions on production and importation of GM crops. GM crop adoption affects producers and consumers, not only through technological change, but also through trade policy responses. This article reviews open economy analyses of impacts of GM crops. To varying degrees, commodities are segmented into GM, conventionally grown, and organic product markets. Recent advances in trade modeling consider the consequences of market segmentation, along with consequences of GM crop import restrictions, product segregation requirements, and coexistence policies.