I. Introduction

The precautionary principle has become an increasingly important component of environmental policy, considered by the European Union (EU) to be a "full-fledged and general principle of international law."¹ As the precautionary principle has gained prominence, policy analysts have devoted increasing attention to the issue of implementation.² Nevertheless, the practical implications of the principle remain unclear. The difficulty of formulating environmental policy pursuant to the principle is reflected in the European Commission's recent communication concerning the principle.³ Even European commentators acknowledge it is "doubtful that the communication will go far in clarifying the EU's use of the principle or convincing the [United States] of its validity."⁴

The vagueness of the principle explains why it so hard to implement. Consider the version of principle invoked in § 15 of the Rio Declaration on Environment and Development: