The paper aims at stimulating reflection and debate on the issue of uncertainty as a key component of contingency planning and crisis management in environmental matters. It identifies and discusses different types of uncertainty, besides the scientific one which is already widely appreciated. A diagnostic tool is presented in the form of a checklist, designed for detecting and ranking the different types of uncertainty and for assessing levels of severity in a given contingency or crisis. The checklist is subsequently applied to the analysis of a classical disaster (the Seveso accident) to evaluate its usefulness and effectiveness. The author calls for further testing of the checklist by researchers, planners, regulators and practitioners and welcomes advice for its refinement.