Mechanisms that may support magnetic-field-aligned electric fields in collisionless plasma are discussed in the light of recent magnetospheric observations which for the first time allow a quantitative test of the theoretical models. Data from barium ion releases which indicate large field-aligned potential drops and direct electric field probe measurements at high altitude which reveal electric fiels of several hundred millivolts per meter are discussed. It is concluded that the large field strengths observed (1) cannot be explained by anomalous resistivity or thermoelectric effects based on wave-particle interaction, (2) are much larger than required merely to balance the local mirror forces, and (3) are compatible with electric double layers of the same nature as those observed in the laboratory.