The calculation of electrical conductance from the transmissive behavior of the sample has become a widely accepted procedure. This discussion visits a number of points where, despite this acceptance, the increasing use of formal techniques has obscured elementary physical considerations. For a simple planar tunneling barrier the exact applicable formula depends on the geometry of the leads. We stress the role of inelastic transitions in the leads for samples where transmission depends sensitively on energy. The difficulties in extending the approach, which calculates conductance from transmission, to high frequencies, are explained. We also emphasize that the single electron transmission probability, and the occupation probability in the emitting reservoirs, without further regard to the exclusion principle, define the conductance even in the presence of a time-dependent scattering potential. Spatial variations of current and field, due to localized scatterers, are revisited only briefly, to point to further needed calculations. © 1992 IOP Publishing Ltd.