1. Potassium channels, which control cell electrical activity, are among the most regulated of all ion channels in biology. Promotion of activity in K+ channels by a wide range of physiological factors tends to stabilize cell function. 2. The discovery of synthetic molecules (e.g. cromakalim) that 'directly' open ATP-sensitive K+ channels has led to a new direction in pharmacology. ATP-sensitive K+ channel-opening properties have subsequently been demonstrated in a diverse range of chemical structures (synthetic and endogenous). 3. The existence of so many different subtypes of K+ channels has been an impetus in the search of new potassium channel openers with different channel selectivities and thus biological profiles. 4. The decrease in cell excitability following K+ channel opening implies a broad clinical potential in a number of pathological conditions for K+ channel openers. Preclinical and clinical evidence supports therapeutic roles of K+ channel openers in disorders of a wide range of biological cells. 5. Although lack of selectivity of current compounds remains a major hurdle, advances in K+ channel openers and K+ channel pharmacology are encouraging. Differences already observed in the pharmacology of K+ channel openers are important factors for the development of second-generation compounds, when tissue selectivity is sought. 6. The availability of subtype-selective K+ channel openers will facilitate detailed study, through a combined effort of electrophysiology, functional pharmacology and molecular biology, leading to focused therapeutic approaches for defined pathological conditions.