In its restless metamorphosis, the environmental movement captures ideas and transforms them into principles, guidelines and points of leverage. Sustainability is one such idea, now being reinterpreted in the aftermath of the 1992 Rio Conference. So too is the precautionary principle. Like sustainability, the precautionary principle is neither a well defined principle nor a stable concept. It has become the repository for a jumble of adventurous beliefs that challenge the status quo of political power, ideology and civil rights. Neither concept has much coherence other than it is captured by the spirit that is challenging the authority of science, the hegemony of cost-benefit analysis, the powerlessness of victims of environmental abuse, and the unimplemented ethics of intrinsic natural rights and inter-generational equity. It is because the mood of the times needs an organising idea that the precautionary principle is getting a fair wind. However, unless its advocates sharpen up their understanding of the term, the precautionary principle may not establish the influence it deserves. Its future looks promising but it is not assured.