The solar wind flows past the earth and stretches the geomagnetic field behind it, forming a tail to the magnetosphere. Magnetic-field lines are directed toward the sun in the upper half of the tail and oppositely directed in the lower half. The separation region where the field reversal takes place is occupied by an enhanced plasma flux, which keeps the oppositely directed fields from annihilating themselves. Satellite observations have also found intense transient electron fluxes within the plasma sheet region. Although the role of the tail in the magnetic substorm phenomena is not yet clear, during the main phase of a magnetic storm the tail appears to contribute significantly to the dynamical distortion of the magnetosphere, the decrease of the horizontal geomagnetic field on the earth's surface, and the lowering of the latitude of the trapping boundary. Although the tail shows a well-defined theta geometry at 80 RE, lunar orbit = 60 RE, it has not been clearly observed at 500 and 1000 RE. A radial gradient of the magnetic field magnitude is consistent with merging occurring at the neutral sheet and with the tail length being of the order of 100–200 RE. The basic mechanism for terminating the tail, as well as the means for momentum transfer on its boundary forming the tail, are not yet established. This paper reviews past experimental and theoretical models of the geomagnetic tail, outlines our present knowledge, and identifies the current major problems associated with its dynamics.