The implantable cardioverter defibrillator (ICD) has been in clinical use for 20 years and its clinical role is becoming increasingly clear. A number of well-designed trials demonstrated its effectiveness in high-risk patients who have already experienced a malignant arrhythmia. A more controversial role for the ICD is in patients who are at high risk but have not yet had an arrhythmic event. Randomized clinical trials published in the late 1990s demonstrated survival benefit with the ICD in narrowly defined high-risk populations. These populations are presently defined by a low ejection fraction and inducible ventricular tachyarrhythmia. Clinical trials still in progress will determine whether broader populations will benefit from prophylactic ICD implantation. These trials will have broad clinical importance.