SK channels play a fundamental role in all excitable cells. SK channels are potassium selective and are activated by an increase in the level of intracellular calcium, such as occurs during an action potential. Their activation causes membrane hyperpolarization, which inhibits cell firing and limits the firing frequency of repetitive action potentials. The intracellular calcium increase evoked by action potential firing decays slowly, allowing SK channel activation to generate a long-lasting hyperpolarization termed the slow afterhyperpolarization (sAHP). This spike-frequency adaptation protects the cell from the deleterious effects of continuous tetanic activity and is essential for normal neurotransmission. Slow AHPs can be classified into two groups, based on sensitivity to the bee venom toxin apamin. In general, apamin-sensitive sAHPs activate rapidly following a single action potential and decay with a time constant of approximately 150 ms. In contrast, apamin-insensitive sAHPs rise slowly and decay with a time constant of approximately 1.5 s. The basis for this kinetic difference is not yet understood. Apamin-sensitive and apamin-insensitive SK channels have recently been cloned. This chapter will compare with different classes of sAHPs, discuss the cloned SK channels and how they are gated by calcium ions, describe the molecular basis for their different pharmacologies, and review the possible role of SK channels in several pathological conditions.