Potassium channels play an essential role in the membrane potential of arterial smooth muscle, and also in regulating contractile tone. Four types of K⁺ channel have been described in vascular smooth muscle: Voltage‐activated K⁺ channels (K_V) are encoded by the Kv gene family, Ca²⁺‐activated K⁺ channels (BK_Ca) are encoded by the slogene, inward rectifiers (K_IR) by Kir2.0, and ATP‐sensitive K⁺ channels (K_ATP) by Kir6.0 and sulphonylurea receptor genes. In smooth muscle, the channel subunit genes reported to be expressed are: Kv1.0, Kv1.2, Kv1.4–1.6, Kv2.1, Kv9.3, Kvβ1–β4, slo α and β, Kir2.1, Kir6.2, and SUR1 and SUR2. Arterial K⁺ channels are modulated by physiological vasodilators, which increase K⁺ channel activity, and vasoconstrictors, which decrease it. Several vasodilators acting at receptors linked to cAMP‐dependent protein kinase activate K_ATP channels. These include adenosine, calcitonin gene‐related peptide, and β‐adrenoceptor agonists. β‐adrenoceptors can also activate BK_Ca and K_V channels. Several vasoconstrictors that activate protein kinase C inhibit K_ATP channels, and inhibition of BK_Ca and K_V channels through PKC has also been described. Activators of cGMP‐dependent protein kinase, in particular NO, activate BK_Ca channels, and possibly K_ATP channels. Hypoxia leads to activation of K_ATP channels, and activation of BK_Ca channels has also been reported. Hypoxic pulmonary vasoconstriction involves inhibition of K_V channels. Vasodilation to increased external K⁺ involves K_IR channels. Endothelium‐derived hyperpolarizing factor activates K⁺ channels that are not yet clearly defined. Such K⁺ channel modulations, through their effects on membrane potential and contractile tone, make important contributions to the regulation of blood flow.