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The ionosphere, even in its "undisturbed" state, is constantly in motion. Its charged and uncharged constituents may travel together or independently, while irregularities in the distribution of charge may take a different course again. The pertinent observational data are extensive and, in some cases, conflicting; their interpretation is seldom direct. The theoretical factors affecting that interpretation are becoming clear, though in some cases their areas of relevance are still subject to divergent opinions. An attempt is made here to bring into focus the theoretical factors themselves, to record their bearing on the observations wherever it has been established, and to suggest paths of future progress wherever it has not. Details of observational results and theoretical developments are largely suppressed, and with them the conflicts and discrepancies which, however real, would obscure the presentation of the principles involved.