Abstract 1. 1. Rapid deployment of military forces to extreme naturally harsh environments presents combatants with physiological, biological and psychological stresses that: (1) affect mood, motivation, activation; (2) result in decreased job performance; (3) make accidents more likely; (4) lead to illnesses and non-battle injuries; and (5) jeopardize mission success 2. 2. Laboratory and field research help develop countermeasures to assist military forces in adjusting to hot dry desert, hot wet jungle, cold arctic regions, and high terrestrial altitudes 3. 3. Proven preventive medicine techniques and advanced concepts for deployment preparation include: pre-acclimatization, anti-jet lag measures, hydration, selection of rations based on nutrition requirements, pharmacological intervention, field sanitation, work-rest-sleep scheduling, and controlling operational stresses.