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Abstract : Numerous phenomena observed at the surface of theearth, in the magnetospheric cavity, and in the neighborhood of its boundary are related to the flow of the solar wind. The extent to which this flow can be analyzed by the techniques of continuum fluid mechanics is discussed in relation to the observational and theoretical evidence. Numerous calculations have been made to determine the shape of the magnetosphere cavity, the location of the bow shock, and the variations in flow properties within the shock layer. Observations of auroral motions and alignments have indicated that flow patterns with appreciable velocities also exist inside the magnetosphere, thus suggesting appreciable coupling across the boundary. Analysis of such a boundary leads to a rate of reconnection of the field lines which decreases only logarithmically with the magnetic Reynolds number. The logarithmic dependence implies a rapid rate of reconnection which is insensitive to uncertainties in the effective conductivity of the plasma. Applying this result to the flow over the magnetosphere, the conclusion is that more than 10% of the magnetic field lines brought to the surface of the magnetosphere by the solar wind get broken and reconnected to the earth's polar field lines. (Author)