The nature of uncertainty in coastal engineering is discussed with reference to an empirical study of practising coastal engineers in the UK. This study provided evidence which indicates that a complex multi-disciplinary set of issues are taken into account in decision making only some of which are explicitly articulated. It is argued that current approaches to handling uncertainty based on reliability theory provide an important but incomplete perspective on a rich and complex topic. An approach to uncertainty modelling based on Interval Probability Theory is proposed. Process modelling is used to track the sources of uncertainty in analysis, design and decision making and to integrate different types of evidence. This approach to uncertainty analysis, which involves exploring and accounting for the sources and nature of uncertainty in decision making, is illustrated with reference to sea defence projects on the East Coast of the UK.
