Changes in intracellular Ca²⁺ are central to the function of smooth muscle, which lines the walls of all hollow organs. These changes take a variety of forms, from sustained, cell-wide increases to temporally varying, localized changes. The nature of the Ca²⁺ signal is a reflection of the source of Ca²⁺ (extracellular or intracellular) and the molecular entity responsible for generating it. Depending on the specific channel involved and the detection technology employed, extracellular Ca²⁺ entry may be detected optically as graded elevations in intracellular Ca²⁺, junctional Ca²⁺ transients, Ca²⁺ flashes, or Ca²⁺ sparklets, whereas release of Ca²⁺ from intracellular stores may manifest as Ca²⁺ sparks, Ca²⁺ puffs, or Ca²⁺ waves. These diverse Ca²⁺ signals collectively regulate a variety of functions. Some functions, such as contractility, are unique to smooth muscle; others are common to other excitable cells (e.g., modulation of membrane potential) and nonexcitable cells (e.g., regulation of gene expression).