This chapter describes existing measures of injury severity, reviews the rationale for their development, and discusses their use and important limitations. Scoring systems based upon the anatomic injury are an important method of measuring injury severity and are critical to injury epidemiology, injury prevention, trauma system development and outcome analysis. In addition to the information captured by scales of anatomic injury severity, measures of the physiologic response to injury such as heart rate (HR), systolic blood pressure (SBP), respiratory rate (RR), and level of consciousness are also important indicators of the severity of an injury. By combining measures of the severity of anatomic injury with those of acute physiologic derangement and age, it has been possible to provide statistically strong estimates of survival probability following trauma. Finally, the chapter also reviews the measures of co-morbidity, the term used to describe health status factors likely to influence the outcome after injury.