Birkeland currents are present at both Earth and Mercury and the differences in currents reflect fundamental differences in the convection dynamics of these systems. At Earth, the conductive ionosphere and the high altitude magnetosphere are linked with these currents through the Pedersen conductance which closes the currents in the ionosphere and through magnetospheric plasma that carries the Region 2 currents between the evening and morning sectors, shielding the inner magnetosphere from the convection potential. The currents reflect the convection of magnetic flux from the dayside, over the polar caps into the magnetotail, and subsequently sunward in the tail to the dayside. At Mercury, Birkeland currents close through the planet itself with at least half of the current reaching the core to close laterally at polar latitudes. There is no evidence of Region 2 currents at Mercury implying that the magnetosphere and planet itself are not shielded from the convection cycle and suggesting that the planet also participates in the return convection although how this is actually accomplished is not known.