Human ether-a-go-go-related gene (hERG) potassium channels conduct the rapid component of the delayed rectifier potassium current, IKr, which is crucial for repolarization of cardiac action potentials. Moderate hERG blockade may produce a beneficial class III antiarrhythmic effect. In contrast, a reduction in hERG currents due to either genetic defects or adverse drug effects can lead to hereditary or acquired long QT syndromes characterized by action potential prolongation, lengthening of the QT interval on the surface ECG, and an increased risk for "torsade de pointes" arrhythmias and sudden death. This undesirable side effect of non-antiarrhythmic compounds has prompted the withdrawal of several blockbuster drugs from the market. Studies on mechanisms of hERG channel inhibition provide significant insights into the molecular factors that determine state-, voltage-, and use-dependency of hERG current block. In addition, crucial properties of the high-affinity drug binding site in hERG and its interaction with drug molecules have been identified, providing the basis for more refined approaches in drug design, safety pharmacology and in silico modeling. Recently, mutations in hERG have been shown to cause current increase and hereditary short QT syndrome with a high risk for life-threatening arrhythmias. Finally, the discovery of adrenergic mechanisms of hERG channel regulation as well as the development of strategies to enhance hERG currents and to modify intracellular hERG protein processing may provide novel antiarrhythmic options in repolarization disorders. In conclusion, the increasing understanding of hERG channel function and molecular mechanisms of hERG current regulation could improve prevention and treatment of hERG-associated cardiac repolarization disorders.