Abstract This review will give an overview of the current state of knowledge about the structures of ion channels; the mechanisms of selectivity and gating; the cell biology of ion channels, including coassembly of channel subunits with other proteins and trafficking of ion channels; the physiological functions of ion channels and their roles in disease; and pharmacological tools available for the manipulation of ion channels in vitro and in vivo. While a vast array of ion channels exist with different selectivities and mechanisms of activation, some of these channel classes are reviewed in other chapters of this edition, and this chapter will focus on voltage‐gated K + , Na + , and Ca 2+ channels.