A large number of severity of illness scoring systems have been developed and they are widely used in intensive care practice. However, they are complex systems with their basis in mathematics. To use such systems effectively, it is important to appreciate what factors influence their performance so that they can be compared fairly and used most appropriately. The purpose of this review is to describe the methods commonly used to assess the various facets of performance in severity of illness scoring systems. The performance of the most frequently used scoring systems in adult intensive care practice are presented. The shortfalls, misuse and strengths of scoring systems are also discussed.