An implantable defibrillator looks very similar to a pacemaker. While most defibrillators can also function as pacemakers, their main role is to deliver energy in the form of a shock to bring the heart out of a serious rhythm disorder.

A defibrillation system has two main parts: the defibrillator and the leads. The defibrillator is a small metal device containing electronics and a battery. The leads are thin wires that sense the heart's rhythm and deliver the electrical signal to the heart when directed to do so by the defibrillator.