Electrical excitability is a fundamental property of the neuromuscular systems of metazoans. The varied response of neurons to electrical excitation is largely accounted for by a diverse set of voltage-gated potassium (KV) channels in the excitable membrane. The complete structure of a KV channel is not yet available. However, recent structural biological experiments have begun to provide new insight into how specific KV channels are formed and regulated, and how they function and interact with other proteins. In particular, the selectivity of KV channels for K+ and suggestions as to how these structural elements might assemble into a functional KV channel are discussed.