Voltage-dependent K(+), Ca(2+), and Na(+) channels play vital roles in basic physiological processes, including management of the action potential, signal transduction, and secretion. They share the common function of passively transporting ions across cell membranes; thus, it would not be surprising if they should exhibit similarities of both structure and mechanism. Indeed, the principal pore-forming (alpha) subunits of each show either exact or approximate 4-fold symmetry and share a similar transmembrane topology, and all are gated by changes in membrane potential. Furthermore, these channels all possess an auxiliary polypeptide, designated the beta subunit, which plays an important role in their regulation. Despite considerable functional semblences and abilities to interact with structurally similar alpha subunits, however, there is considerable structural diversity among the beta subunits. In this review, we discuss the similarities and differences in the structures and functions of the beta subunits of the voltage-dependent K(+), Ca(2+), and Na(+) channels.