Two decades after the first genetically engineered (GE) seeds became commercially available for major field crops, GE varieties have been widely adopted for U.S. corn, soybean, cotton, canola, and sugar beet production. The small, longstanding market for organically grown food (which excludes GE seed and material) continues to expand and a market for conventionally grown foods produced without GE seed has also emerged. In order to maintain the integrity of GE-differentiated markets, organic farmers—and other farmers using non-GE seeds—employ a variety of practices to avoid the accidental mixing of GE material in their crops. This report examines organic and conventional product markets in the United States. It describes commonly used coexistence practices and discusses the economic impacts when GE material is detected in organic crops.