The document provides guidance to those concerned with the safety of human exposures to extrem hot or cold thermal environments, about the medical fitness assessment and health monitoring which may be appropriate prior to and during such exposures. It is intended to assist those with responsability for such exposures to reach decisions about the appropriate level of medical supervision in different situations. The document presents guidance which should be read and used in context of other guidance and legislation applying to each particular situation. Extreme thermal environments are those which result in a high rate of heat gain or loss by the body. A precise definition of such environments cannot easily be given, as the change in body heat storage depends on clothing and activity as well as the parameters of the climatic environment. As a guide, the boundaries of extreme environments might be considered to be as follows: for hot environments, a wet bulb globe temperature of 25 °C; for cold environments an air temperature of 0 °C or below. This standard defines the following terms: independent medical officer, experimenter, medical fitness assessment, health monitoring, occupational physician, principal investigator.