There has been considerable recent interest in the relationship between the cross-tail magnetic field component By and tail dynamics. The purpose of this paper is to give an overall description of the penetration of the interplanetary magnetic field (IMF) By into the near-Earth plasma sheet. We show that plasma sheet By may be generated by the differential shear motion of field lines and enhanced by the flux tube compression. The latter mechanism leads to a By analogue of the pressure-balance inconsistency (Erickson and Wolf, 1980) as flux tubes move from the far tail toward the Earth. The growth of By, however, may be limited by the dawn-dusk asymmetry in the shear velocity as a result of plasma sheet tilting. By penetration into the plasma sheet implies field-aligned currents flowing between hemispheres. These currents together with the IMF By related mantle field-aligned currents effectively shield the lobe from the IMF By. ¿ American Geophysical Union 1995