The term ectopic activity is used to indicate situations in which the electrical stimulus responsible for the contraction of the heart originates in a zone of the cardiac tissue (ectopic focus) different from the normal one (the sinoatrial node). Two main mechanisms exist that may induce ectopic activity: automaticity (generally subdivided into normal and abnormal) and triggered activity. Ectopic activity caused by normal automaticity appears in cardiac cells in which the automaticity (spontaneous generation of electrical impulses) is an intrinsic property. In normal situations, the potential pacemaker activity in cells different from those in the sinoatrial node is inactive because the spontaneous frequency of impulse generation is higher in the sinoatrial node. However, when the frequency of the sinoatrial node decreases below the spontaneous frequency of a subsidiary pacemaker, this pacemaker takes the control of the heart rate and becomes an ectopic focus. Under several pathological situations, any cell in the heart, even those that normally have no spontaneous activity, could become abnormally automatic. Another mechanism for ectopic activity is known as triggered activity, which consists of an impulse (or a train of impulses) provoked by after-depolarizations, which are abnormal oscillations of the action potential. Two types of after-depolarizations exist: early (EADs), which appear during the repolarization phase of action potential, and delayed (DADs), which occur when the repolarization phase of the action potential is completed. The main characteristic of the ectopic activity induced by after-depolarizations is that a previous electrical impulse is necessary to trigger one or more ectopic beats.