Human thermoregulation is described in relation to the need to: maintain thermal balance with the environment while carrying out critical and generic tasks; maintain skin temperatures in a range defined as comfortable; provide fluid to prevent dehydration; and ensure that ventilation should be sufficient to support human health and performance. Basic environmental terminology is described including the WBGT index and relative humidity. Heat acclimatization is described and guidance is given for the design of work in hot or cold climates. The Bedford Scale for assessing thermal comfort is provided. There is a discussion of air quality, ventilation, and “sick” buildings, together with a scale for assessing perceptions of air quality. Basic guidance for heat stress assessment is given. The chapter ends with a discussion of the relationship between thermal environment, performance, and productivity.