This chapter on scoring system in ICU. Scoring systems involve collection of data regarding a patient based on clinical and laboratory records; these collected data variables are assigned points. APACHE, SAPS, MPM, SOFA, MODS, LOD and TRIOS are severity illness scoring systems have become important tools for the evaluation and planning of intensive care practice patterns. They are objective tools used as a group measurement for patients with similar disease process; however, they are not suited for individual use. Data collection and entry errors, misapplications of the model, confidence interval not reported, computational errors, failure to account for sample size, chance variability, determination of diagnosis prone to bias, lead time bias, inclusion of ineligible patients and wrong diagnosis selected are the various pitfalls in application of scoring systems. Precisely define outcomes of interest, identify and define candidate predictor variables, examine continuous variables, perform univariate analysis, perform multivariate analysis, examine and adjust for interaction between variables, test calibration of model and test discrimination of model are steps in developing a scoring system model