The precautionary principle applies to control risks the existence of which is uncertain under well-established scientific standards, mostly because generally accepted scientific knowledge is not or is not yet available. As such, the principle has been frequently relied upon to protect markets against possibly unsafe goods on the ground that, in view of the scientific uncertainty, it is better to err on the side of caution. Although being suspected of sometimes serving protectionist goals, the principle has increasingly been recognized in international law and in European Community law. The report, therefore, examines the status of the principle under public international law, in particular as regards the international conventions on the protection of the environment and the agreements concluded within the framework of the WTO, where the principle has been relied upon in several trade disputes. This examination is then contrasted with the development, which the precautionary principle has taken under Community law both in the case-law of the Court of Justice and in the regulatory and administrative practice of the Commission. A particularly interesting field of application of the principle concerns its use as a justification under the safeguard or exception clauses of harmonisation of laws. The result is that the principle has matured into a self-contained justification of exceptions to free intra-Community trade, albeit one that is surrounded by a number of limiting rules. This becomes clear from the Commission’s Communication on the Precautionary Principle, as well as from the way the principle has been enshrined and applied in EU law. Another interesting feature is how intra-Community development of the principle ties in with its international development, since the Community is bound to use the principle as a defense when justifying protective regulatory action on the international level while, on the internal level, it must limit the application of the principle with a view to implement a free trade regime. Similarly, it is noteworthy how the precautionary principle has come to be recognized in a globalized world economy, and how it is handled or must be handled so as to reconcile the interest in human and animal safety or the protection of the environment with innovation and access to markets.