This chapter introduces an approach that makes use of the well‐established black lipid membrane (BLM) technique. The advantages of such an approach are convenient application of electrical measurements, direct access to the aqueous compartments on both sides of the membrane, and the possibility of investigating the functions of a reconstituted protein under well‐controlled conditions such as lipid composition of the membrane and the choice of the electrolytes. The BLM serves as a “capacitive electrode,” and couples the charge transfer in the protein to a compensatory charge movement between both compartments separated by the BLM. A crucial condition of the gated‐channel concept is the existence of two strictly regulated gates that encase the ion‐binding sites. The equivalence of current‐relaxation and admittance techniques was first shown in studies of the kinetics of passive ion transport through a BLM in the presence of ionophores or hydrophobic ions.