The patch-clamp technique allows the measurement of current through a wide variety of channels under reasonably realistic conditions, while controlling (“voltage clamping”) one component of the driving force for current, the electrical potential. The other component of the driving force is set by the concentrations of permeant ions on both sides of the membrane, and those need to be controlled as well if the function and mechanism of channels are to be studied. In natural biological settings, current through channels is determined as much by chemical messengers, metabolites, modulators, and drugs as by driving force, and these must be applied to one side of the membrane or another if their action is be understood.