Recent decades have seen great progress in computer modeling of the global thermosphere-ionosphere system. The models are derived by solving simultaneously the conservation equations of mass, momentum, and energy that govern the concentrations, velocities, and temperatures of the ions, electrons, and neutral gases. Typically they cover heights from 80–100 to 500–700 kilometers, with resolutions of 100–1000 kilometers horizontally, 10 kilometers vertically, and tens of minutes in time; they do not deal with smallscale structure such as plasma instabilities. Some models extend down into the mesosphere (30–80 kilometers) or up to the protonosphere, the region where the dominant constituents are thermal protons and electrons. The input parameters include photochemical and transport rate coefficients and the fluxes of solar ionizing radiation and energetic particles, and the models can be run for general levels of solar and geomagnetic activity or for specific dates. Calibrated with real data, the models mimic well the behavior of the real thermosphere and ionosphere and have contributed much to understanding, besides helping the practical task of ‘near-real-time modeling,’ which is important for communications. They differ from purely descriptive ‘empirical’ models, which merely seek to describe the upper atmosphere and ionosphere, not to explain it.