Voltage-dependent conductances play a fundamental role in the generation and propagation of the action potential. Hodgkin and Huxley (1952c) made a complete description of the membrane currents underlying the generation of the impulse. Their description introduced the concepts of separate pathways for sodium and potassium (today called Na and K channels) and the idea of conductances modulated by the membrane potential. Our present view of the sodium and potassium conductances are based on the idea that the membrane has discrete conducting units called channels. These units are macromolecules highly specialized for ion conduction and embedded in the lipid matrix where they can sense the electrical field across the membrane and respond to its changes. This article is concerned with the mechanisms by which ion conductances depend on membrane potential, a process called voltage gating. Our knowledge of voltage gating is based on measurements of macroscopic currents, analysis of current noise, single channel recordings and gating currents. We will first examine the relationship between macroscopic and microscopic determinations and their relation to gating currents and then results of the different measurements will be analyzed in terms of models. There are many recent reviews on voltage gating with varying emphasis on the different types of experimental results (Cahalan, 1980; Armstrong, 1981; French & Horn, 1983). This review will emphasize results on gating currents, which are a direct expression of the voltage-dependent process. These currents are small and their detection requires subtraction of large currents. Consequently,