This paper contrasts two paradigms of environmental regulatory decision making, "permissive regulation" and "preventative design," with respect to their treatment of scientific and legal uncertainty and the allocation of legal standards and burdens of proof. "Permissive regulation," which is the predominant approach in Canada, suffers two types of statistical errors. A type I error occurs when, for example, a pollution control device is unjustly imposed on an industry. A type II error occurs when no action is taken to control an industry when, in fact, damage is taking place. Concern to prevent type I errors often leads to type II errors. Attempts to resolve these problems through incremental changes in legislation and policy have generally failed. This article illustrates the scientific and regulatory problems associated with 'permissive regulation" through an analysis of environmental common law, legislation, and regulation. Protection of environmental quality requires regulatory decision making rooted in the principles of precautionary, preventative action that tends to minimize costly type II errors. With the "preventative design" approach now being used in several jurisdictions, the regulatory burden of proving harm is shifted from regulators to the polluters who must demonstrate safety. European and American initiatives as well as international agreements illustrate the historical development and implementation of this "preventative design" perspective. This article suggests that Canadian legislation and regulations be written with this approach.