This thesis approaches two theoretical problems associated with the plasma sheet in the geomagnetic tail. The first problem is the structure of the plasma sheet. It is approached by considering the plasma sheet as a sheet of collision-less plasma with isotropic gas pressure that is in a state of balance between the macroscopic forces. Only the forces due to the plasma gas pressure and the magnetic field forces are considered. The calculations are done in two dimensions by neglecting any Ysm component of the geomagnetic tail field. The discussion of force balance argues that, unless there is a significant contribution to the forces in the tail from some neglected source, the normal component of field must be on the average positive in all parts of the plasma sheet. A model configuration is computed as an example of force balance and yield the relation Bzo /(dp * Bxx) = 0.72 where Bzo is the normal component of field in the center of the sheet (gammas), dp is the sheet half thickness (Re), and Bxx is the radial gradient of the external field strength Also, the z-variation of the plasma sheet field strength show no presence of a neutral sheet, a thin (~10 to the 3rd km) reversing layer with a field strength of several gammas on either side, within the plasma sheet. Indeed, a neutral sheet may be inconsistent with a force balance plasma sheet unless additions balancing forces or anisotropies of the plasma pressure are considered. The second problem is a study of the effect of a geomagnetic-like neutral sheet, a thin current sheet where ionic, and maybe electronic, motions are non-adiabatic, on plasma. The results of previous studies by Speiser are found to be basically correct but limited. It is shown that the energy, pitch angle and phase angle dependence of the change in pitch angle of ions traveling through the neutral sheet makes it unlikely that the neutral sheet is responsible for direct precipitation of plasma sheet ions into the auroral ionosphere as suggested by Speiser, The effective conductivities of the neutral sheet needed to describe the neutral sheet with MHD are found to be useful but accurate only as a rough estimate of the time effective conductivity which may be a function of position and other parameters.