The purpose of this handbook is to help activists effectively protect the consumer’s right to know about products that have been genetically modified from challenges based on the WTO trade rules. Unless groups representing the public interest become vocal participants in discussions about GMOs and the world trading system, it is likely that WTO rules will be interpreted according to industry preferences. To enable activists to participate more effectively in the GMO debate, this handbook examines the relationship between GMO labeling schemes and the relevant rules of the WTO. After considering possible conflicts between these trade rules and GMO labeling schemes, it suggests interpretations of WTO rules that allow GMO labeling schemes to operate without interference from the international trade system. This handbook shows activists how to argue that WTO rules are not an impediment to effective, mandatory GMO labeling.