Background and methods: The development of the Physiological and Operative Severity Score for the enUmeration of Mortality and morbidity (POSSUM) is described and its methods of analysis and value in a modern surgical practice are reviewed. A computerized search of all published data in Medline, the Cochrane Library and Embase was made for the last 12 years. Relevant articles were then searched manually for further papers on risk analysis, case-mix comparison and POSSUM methodology. Results and conclusion: POSSUM has been evaluated extensively in both general and specialist surgery. While there are problems with both data collection and analysis, when used correctly POSSUM can usefully compare outcomes between surgeons and between hospitals. In specialist surgery, individual regression equations may be needed for each index procedure.