Severity scoring systems are the most important adjuncts of treatment used in the intensive care unit to predict outcome, characterize disease severity, degree of organ dysfunction, and assess resource use. Even though disease severity scores are not the key elements of treatment, however, they are an essential part of improvement in clinical decisions and in identifying patients with unexpected outcomes. In fact, they have become a necessary tool to describe ICU populations and to explain differences in mortality. However, it is also important to note that the choice of the severity score scale, index, or model should accurately match the event, setting or application of such systems can lead to wastage of time, increased cost and poor science. Importantly, the different types of scores should be seen as complementary, rather than competitive and mutually exclusive, proper application of severity scores helps in decision making at the right time and in decreasing hospital cost. This review article provides a brief overview of intensive care unit severity scoring systems along with the prediction of death or survival rate calculations, although the article focused on Acute Physiology and Chronic Health Evaluation (APACHE), Simplified Acute Physiology Score (SAPS) and Sequential Organ Failure Assessment (SOFA).