This paper presents modeling methodology and results for a study of Soldier effectiveness in a hot environment. The effectiveness of Soldiers is diminished under conditions of high heat stress. Excessive heat stress will degrade mental and physical performance capabilities and eventually cause heat casualties. The core temperature of a human body provides the “best” single physiological measure to estimate physical work capabilities during hot weather operations. Prediction of Soldier effectiveness in extreme environments can be accomplished through the use of segmental human thermoregulation models. Differences in physiological characteristics among Soldiers can affect thermoregulatory response and must be accounted for when predicting effectiveness. Additionally, prediction accuracy can be improved by combining human thermoregulatory models with a complete characterization of the thermal environment. Human thermal models representing Soldiers with significant physiological differences among them were placed into a full-vehicle thermal HVAC predictive model of a Mine Resistant Ambush Protected (MRAP) vehicle. The simulation was performed to support US Army PMMRAP in its effort to improve Soldier effectiveness under conditions typically encountered on MRAP vehicles fielded in Iraq.