The Australian Defence Force (ADF) has long battled in hot and arduous environments. From the trenches of Tobruk during World War II and the thick jungle of Vietnam through to current theatres of war such as the rugged mountains of Afghanistan. Australian soldiers are exposed to the constant threat of heat illness, and while avoidable in training settings, it cannot be prevented in some combat situations. Prevention however, is a key factor in the exclusion of heat illness cases, a matter that the ADF has worked hard to highlight through mandatory annual training. It is for this reason that military heat casualty rates remain low in the ADF. This article provides some useful assessment tools which help to differentiate between the different levels of heat illness; and furthermore, underlines the critical stages of heat illness and includes literature on the aetiology, pathophysiology, clinical manifestations and prehospital care management of heat illness.