Clinical examination is non-invasive, but has well-recognized limitations in detecting compensated and uncompensated low flow states and their severity. This paper describes the principles of near infra-red spectroscopy (NIRS) and the basis for its proposed use, in hypovolaemic, cardiogenic and septic shock, for assessing global and regional tissue oxygenation. The vascular occlusion test is explained. Limitations of NIRS, current controversies, and what is necessary in the future to make this technology a part of the initial and ongoing assessment of a patient, are discussed as well. The ultimate goal of such techniques is to prevent miss-assessment and inadequate resuscitation of patients, two major initiators in the development of multisystem organ failure and death.