Triggered activity must be added to spontaneous activity and to circus movement as a cause for extrasystoles and tachycardias of either atrial or ventricular origin. The activity of a triggerable focus requires phase 4 depolarization caused by an afterpotential; this distinguishes it from the activity seen in circus movement. A triggerable focus becomes rhythmically active only if driven at a critical rate or by a critically timed premature impulse; this distinguishes it from a focus of spontaneous or automatic activity. The ease of triggering a triggerable focus increases in the presence of catecholamines; triggerable foci in the atrium become quiescent when exposed to acetylcholine. At the present time, fibers within the coronary sinus provide the most persuasive example of triggered activity as a possible cause of arrhythmias of clinical significance. It is possible that the coupled extrasystoles of digitalis toxicity may be triggered; there is every reason to believe that further examples of triggered arrhythmias of possible clinical significance will be discovered.