The Health and Safety Executive (HSE) has considerable interest in predicting the consequence of releases of hazardous liquids from storage tanks (HSE, 1998), because of its responsibilities as a statutory adviser to local planning authorities on the siting of hazardous installations and on the control of development in their vicinity, and because of its responsibilities alongside the Environment Agencies as the Competent Authority in the assessment of COMAH safety reports. As a result, HSE has commissioned a number of studies, including: experimental studies of the behavior of liquid spills (Lee & Kountouris, 1992; Skin Wheeler, 1989; Bentinck Crow, 1991; Clark Savery, 1993; Law & Johnskareng, 1994; Ruddle Widowson, 1985; Ruddle and Elms, 1985; Cleaver, Cronin, Evans, & Hirst, 2001), a review of overtopping data and theoretical models used to study liquid flow (Thyer & Jagger, 1997), the characterization of the profile of the leading edge of the spreading liquid (Thyer, MacMillan, & Jagger, 1999), and the development and use of complex mathematical models to predict overtopping fractions and liquid spill behavior (Daish et al., 1998; Ivings & Webber, 2001).This paper summarizes the consequences of catastrophic tank failure, the current position on the availability of liquid spill data for model validation, and indicates how such data may be used to predict the often considerable fraction of liquid that could overtop retaining bunds around real tanks. Crown Copyright (C) 2002 Published by Elsevier Science Ltd. All rights reserved.