Despite at least 20 years of evolution and with an ever-increasing number of systems, severity of illness quantification remains an imperfect and misused art. To many practicing clinicians the subject remains difficult and is often misinterpreted. Significant discrepancies exist between what these systems actually tell us and what we would like to know. Our aim in this chapter is to highlight these discrepancies and suggest, as have others, that new systems, developed in novel ways, are required if we want reliable answers to certain questions.