Two conditions establish the threshold for protective action in the presence of scientific uncertainty; 1. Credible evidence that a synthetic chemical can cause biological changes that are known to result in unintended harmful outcomes to human health or the environment in some cases; 2. The presence of such a chemical where it does not belong and where it can cause damage to biological systems (such as human bodies). Acting with foresight takes many forms. We must create and strengthen human health and wildlife monitoring programs to detect and predict harm; take steps to prevent, eliminate, and mitigate exposure when credible evidence of harm is found; monitor novel technologies; consider clusters of problems to be early warnings of harm; and open toxic tort records. All action taken must be based on precautionary definitions of “harm” and “credible evidence” and must include public participation. Significant precautionary actions may be taken on the state and local level in advance of a precautionary national chemicals policy.