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The global morphologies of the F1 and F2 regions at magnetically quiet times are reviewed, and attention also is given to the sunspot-cycle variations. The physical conditions, temperature, pressure, recombination coefficients, and collision frequencies are reassessed in the light of recent studies of rocket and satellite results and of diffusion. The theory of the F region is reviewed with special attention to Bradbury's hypothesis and to the effects of transport of ionization. Also considered are the morphology of "spread-F" and radio star scintillation phenomena. A theory of the latter is outlined, and it is shown that the undersurface of the F region is unstable at times of upward drift, which appear to be the times when such phenomena are prominent.