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The parameters controlling the development of a simple model of the E and F regions of the ionosphere are discussed. Uses of such models in scientific investigations and engineering applications are given. A computer model is described that will provide estimates of the electron and ion densities in middle latitudes as a function of latitude, longitude, altitude, season, time, and solar activity. Statistical parameters for midlatitude blanketing sporadic E are presented. An analytic expression is used for the CIRA 1965 neutral atmospheric models. The F1 region is treated using a photoequilibrium model with three ionic constituents and 62 ionizing radiation groups. The F2 layer uses theoretical models arranged to fit boundary conditions of NmF2, HmF2 and the electron density at 1000 km. Values of NmF2 are obtained by using the CCIR atlas of ionospheric characteristics. The midlatitude sporadic E layer is treated by presenting three parameters of the layer: the probability of observing sporadic E with a blanketing frequency greater than 0.5 MHz, the average peak electron density when sporadic E is observed, and the standard deviation (assuming log normal distributions for the peak electron density). The restrictions on the model and the availability of data on suitable boundary conditions are discussed. Suggestions for further development of models of the ionosphere are given.