In this chapter we describe technical aspects and experimental potential of the two electrodes voltage clamp (TEVC) electrophysiological approach applied to the Xenopus oocyte-expression system. This technique is addressed to the study of a particular class of expressed proteins, those responsible to drive ion fluxes through the plasma membrane. In fact the voltage-clamp technique provides the most direct and sensitive measurement of the functional properties of ion channels and electrogenic transporters, allowing specific ion currents to be recorded under well-defined voltage conditions and temporal control. Besides the study of the physiological properties of specific ion channels as well as their pharmacological modulation, further applications of the TEVC on oocytes include the possibility to introduce single point mutations in the channel construct and to infer to possible structural aspects and functional involvements of single amino acidic residues. To achieve these results these technique should be strictly tied to basic molecular biology techniques. Recent advance of this technique in drug discovery procedures have been briefly enlightened.