BK channels are potassium ion channels found on the surface of a variety of cell types that are essential for the regulation of several key physiological processes, including smooth muscle tone and neuronal excitability. BK channels are regulated by both membrane voltage and intracellular Ca2+. The channel comprises an integral membrane pore, an integral membrane voltage sensor domain, and a large cytoplasmic region that confers Ca2+ sensitivity. Now Yuan et al. (see the Perspective by Weyand and Iwata) have determined the crystal structure of the cytoplasmic domain of the human BK channel. Four cytoplasmic regions form a gating ring at the intracellular membrane surface with four Ca2+ binding sites on the outer perimeter.