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Electron density data show a strong variation in space, time, and geophysical conditions. Empirical models were established to describe this space-time variation over an 11-year solar cycle. The international reference profiles of URSI and COSPAR provide electron densities in the altitude range 54–1000 km at specific locations and times. Global variations of the F2, F1, and E layers were mapped for long-term predictions of the ionospheric characteristics by the Comité Consultatif International des Radiocommunications and NOAA. On the basis of these analyses, several models of the bottomside ionosphere were constructed covering the whole altitude range down to the D layer. Since the advent of ionospheric satellites the models are being extended to higher altitudes. The Bent model, the model of Ching and Chiu, and the Nisbet model contain the height interval from approximately 100 to 1000 km. These models are presently among the most general setups available. A similar model was recently developed at the University of Bonn. It covers the altitude range from about 60 to 3500 km and is based upon rocket data, ionograms, and satellite observations. A comparison of the latter models shows a reasonable agreement despite their different analytical and observational foundations.