Private voluntary standards such as the International Organization for Standardization’s (ISO’s) 14000 series have played an increasingly important role in encouraging corporations to adopt more sustainable business models on their own initiative and not in direct response to governmentally mandated requirements. ISO standards have a number of benefits, including promoting international uniformity; elevating environmental issues within an enterprise; promoting international trade; and providing a minimal level of environmental performance in countries with less than adequate regulatory infrastructure. Concerns about ISO standards include the relationship to public regulation; and ISO 14001’s essentially procedural, as opposed to performance-based, character. International trade agreements such as NAFTA and the WTO Agreement on Technical Barriers to Trade inject ISO standards into the public policy arena. Because of the structure of these agreements, ISO standards may operate either as a sword—a negative standard used to challenge a domestic regulatory action—or a shield—an internationally agreed reference point that bolsters the legitimacy of a national measure. This Essay examines the potential for ISO standards on eco-labeling to act as swords to attack domestic requirements, and those on life cycle analysis to serve as shields to insulate municipal actions from international challenge in areas such as climate protection.