A review of the theories of light ion flow from the polar cap in their hydrodynamic and evaporative forms is offered. Both types of theories should be able to provide correct treatments of the phenomenon. Some difficulties with the hydrodynamic theory are mainly interpretative and should disappear if the so‐called pressure gradient force term in the equations of motion is recognized as really inertial in nature. An important new insight has been provided recently in an evaporative theory by the realization that the electric field in the exosphere required to balance electron and ion fluxes is quite different from the electric field of gravitational separation usually used in ionospheric theory. However, an evaporative theory that is based on realistic boundary conditions as well as an acceptable electric field remains to be worked out. The situation in the polar ionosphere is sufficiently complex that experimental studies will undoubtedly be needed to establish the actual conditions that exist ther