As technology advances rapidly, so do applications with potential adverse implications on human health. The possible threats include risks that can be substantial, far-reaching and irreversible, and currently available methods of investigation, designed to deal with direct exposure-disease associations, are not always suitable. Growing interest is being paid to health effects that may be the consequence of distal, "upstream" determinants. Considering the complex chain of events that links such determinants with health can be extremely difficult, and exposes severe limitations in science. Thus, there is often a mismatch between what is known and what would be required to inform rational, evidence-based decision making, which is increasingly called for. It has become apparent how production and use of scientific evidence in decision making must be accompanied by precaution, especially in those circumstances, more and more common in recent times, where there is an uncertain possibility that serious health consequences might take place. Several cautionary approaches have been proposed, but the Precautionary Principle (PP) has been the object of especially intense debate in recent years. Developed in the field of environmental health, the PP has been clarified, and has been applied or called for in several instances in public health. Although a unique definition is not available, the principle has been characterised, and criteria for its application have been proposed. However, many questions remain open on general as well as specific issues. In this paper, we address some of the questions that are relevant for the PP to support rational decision making in environment and health and more in general to strengthen its contribution towards human health protection.