From the earliest times, medical students have learnt that the fundamental principle of medical treatment and care is to do no harm. Despite this, iatrogenic conditions have reached epidemic proportions in many parts of the world. The application of the precautionary principle to public health has been less clearly enunciated, but it is at least as equally relevant. Whether we are talking about interventions at the population, area, or group level or our overarching relationship to this planet, which is our habitat, the precautionary principle should frame all policy and action. Global warming and massive system disturbance are the realities for public health in the new millennium. Unless we become much more committed to taking the precautionary principle seriously, there may be no public health task left to do. The human species itself may no longer be there.