Genetically modified organisms (GMOs) in the form of plants, animal feeds, and human foods, inspire heated debate because they involve both high knowledge uncertainty and high ethical concern. Concerns for the integrity and sustainability of the natural environment and as well as the social and economic consequences of allowing the supply of seeds or breeding stock to be controlled by large multinational corporations dominated the debate. This case focuses on the differences between how the United States and the European Union have responded to public opinion, scientific evidence, and industry pressures in creating regulations. In its policy decisions the EU relies very heavily on the "precautionary principle" which mandates avoiding a new activity or technology while its long-term consequences remain unknown. The US, on the other hand, does not rely as extensively on the precautionary principle; most policy decisions are guided by the rule that a new activity may proceed until it is shown to cause significant harm. The controversy was eventually taken to the World Trade Organization for resolution. This case illustrates the origins and consequences of regulatory policies diverging between different countries. It also illustrates the tensions in responding to diverse and strong opinions among citizens, scientists, and business interests when conclusive research results are not available.