Abstract : The 21st century Army will be engaged in numerous joint and expeditionary operations throughout the five regional Combatant Commanders areas of responsibility. Currently, Army forces are deployed globally in more than 120 countries. These regions contain a wide range of natural environments that present unique operational challenges to soldiers and equipment. The distribution of climate, terrain and other environmental factors, and their potential effects on Army operations, must be fully understood. There is a direct geographical and physical relationship between where the Army trains soldiers and tests equipment at U.S. installations to where it will deploy its forces. The conduct of any military enterprise is conditioned by the character of the area of operations--the military operating environment (Palka and Galgano, 2005). Military history and military geography, as well as lessons learned from current operations, provide ample evidence that understanding and adapting to the natural environment is a critical component of operational success. A revised framework for understanding the natural environments of operational areas, and their relationships to U.S. training and testing installations, is presented. This framework the Global Military Operating Environments (GMOE) is developed from a worldwide ecoregional classification system that provides a logical and scientifically based approach to characterizing the spatial distribution of climates and associated environmental factors. The GMOE framework allows for comparison of operational environments across the globe to those found on U.S. Army installations where training of soldiers and testing of equipment take place.