The knowledge of the so called prognostic factors or indicators involved in severe head injury (SHI) is an issue of great interest to make predictions about the future of patients with this pathology. Those indicators constitute the basic elements of the different prognostic formulas or models carried out in order to make predictions in SHI. The mentioned models, therefore, will be constructed by a group of variables (prognostic indicators or factors) and several scales (prognostic scales) that are useful for measuring the final outcome of these patients. In this paper we resume, after an exhaustive review of the literature, the knowledge about the prognostic factors related to SHI. These indicators have been classified as follows: clinical, radiological, physiological, and biochemical. Moreover, we have briefly described the prognostic scales more commonly used in SHI.