The importance of space to the Department of Defense (DOD) mission is evidenced by the recent formation of the U.S. Space Command and the growing number of space DOD systems. Research and development in solar-terrestrial physics conducted and supported by DOD agencies is directed toward the short- and long-term requirements of each agency. However, the understanding, specification, modeling, and prediction of the space environment and of space system/environment interactions is of interest to all of the DOD organizations that are responsible for operating or communicating in space. Research in solar, interplanetary, magneto-sphere, ionosphere, thermosphere, and middle atmosphere physics is thus supported by several DOD agencies. Studies of geomagnetic fluctuations, atmospheric radio noise, and radio wave generation and propagation are also funded. The variability and disturbances of the space environment have the greatest impact on space systems operations; research and development thus tends to focus on the dynamical behavior of the solar-terrestrial regions. Laboratory, ground-based, and space-based experiments are conducted, along with associated analytical and theoretical efforts.