This chapter discusses the mechanisms and etiology of monomorphic ventricular tachycardia (VT). The most common mechanism for monomorphic VT in patients with structural heart disease is reentry. Reentry refers to the concept of circular propagation of an electrical wavefront such that the head of the impulse continuously revolves around an area of either anatomic or functional block (or obstacle) and meets that tail of the wavefront forming the circuit. Initiation, resetting, entrainment and termination are the various characteristics of reentry. Infarct-related scar, bundle branch reentry and interfascicular reentry, and verapamil sensitive VTs are different types of types of reentrant ventricular arrhythmias. Automaticity, abnormal automaticity and triggered activity are also discussed in this chapter.