This chapter gives an overview of the electrophysiologic approach for wide QRS complex tachycardia. Pattern recognition and memorization seem to have become the normal method of learning about cardiac rhythm interpretation. In this chapter, only the monomorphic (MM) forms of wide QRS tachycardia are addressed, particularly of the sustained variety. Excluded is irregular and polymorphic tachycardia. This chapter also covers the narrow versus wide QRS complex, narrow versus wide QRS tachycardia, QRS complex morphology, QRS complex duration, the role of QRS rate for distinction among wide QRS tachycardia, multiple mechanisms/causes in patients with wide QRS tachycardia, intracardiac electrophysiologic studies and specific features of clinically significant wide QRS tachycardia. The basic distinction between a narrow versus wide or broad QRS complex is that of a duration measured from the earliest to the latest ventricular depolarization as recorded on surface electrocardiogram (ECG). The vast majority of wide QRS tachycardia are ventricular and, to a much lesser extent, supraventricular with aberrant conduction.