This paper reviews present understanding of the internal structure of the thin boundary layer, termed the magnetopause, that separates the distorted geomagnetic field in the magnetosphere from the flow of solar plasma in the magnetosheath. The fundamental theoretical concepts of the subject are introduced by considering the structure of the boundary layer that exists when a cold, unmagnetized stream of ions and electrons impinges normally on a vacuum magnetic field. This idealized model indicates how the earth's magnetic field is confined by the impact pressure of the solar wind, the geomagnetic field being terminated by induced shielding currents flowing in the magnetopause. The various idealizations and approximations in this model, such as the assumption of a cold solar plasma and the neglect of the interplanetary magnetic field, are examined critically. The presence of thermal plasma in the magnetosphere modifies the structure of the boundary layer so that the ions penetrate substantially deeper (∼100 km) than the electrons into the geomagnetic field. Complications then arise if the solar plasma has a component of velocity parallel to the confined field, as occurs in the downstream magnetopause that bounds the geomagnetic tail. This additional component of velocity generates electric currents parallel to the magnetic field that may destroy the small‐scale equilibrium of the magnetopause and result in a tangential drag on the geomagnetic cavity. The available experimental information on the magnetopause structure is summarized and related to the theoretical investigations. Although the observations confirm some of the theoretical predictions, much of the detailed theory remains speculative at the present t