K+ channels in the heart shape the cardiac action potential, and many existing drugs can inhibit or activate these currents. In particular, their potential therapeutic benefit has been explored in the treatment and prevention of abnormal heart rhythm. Their use in the management of malignant ventricular arrhythmias has been disappointing and is frequently complicated by proarrhythmia. However, K+ channel-blocking drugs developed for supraventricular rhythm problems, in particular atrial fibrillation, may be more successful. Agents currently in use also have a high incidence of cardiac and other side effects. Thus, the field is moving to a strategy targeting K+ channels that are selectively expressed in the atria, and this is particularly appealing to ameliorate ventricular proarrhythmia. Drugs targeting I(Kur) (K(v)1.5), and to a lesser extent I(KACh) (Kir3.1/3.4), are in various stages of development.