Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 62.The ultimate goal of modeling of the plasma in Earth"s environment is an understanding of the magnetosphere and ionosphere as a coupled global system. To achieve this goal requires a coordinated effort between models applied to different spatial scales. The desire to model this system on a global scale is leading to models which encompass larger and larger regions. The ever-increasing availability of computing resources has allowed models to expand to 2 and 3 dimensions. At the other extreme are the micro-scale processes which transfer energy to individual particles within the global system. As more detailed observations become available the necessity for accurately including such processes in the global models becomes more apparent. Then it becomes a question of how to incorporate the necessary physical processes from all scale sizes into a model of a global system. It now seems clear that such multi-scale scenarios exist where micro-scale processes provide energy to the plasma which flows outward from Earth into the distant magnetotail before returning to the near-Earth regions. The challenge of incorporating all relevant processes into a model of this entire plasma path is a formidable one. The existence of separate models of the separate steps along this pathway leads directly to efforts to fuse models with different scales into a single, self-consistent treatment.