Wide QRS complex is present when the normal activation pattern is modified by various mechanisms and clinical conditions. Correct interpretation is crucial for appropriate decision making. When approaching an electrocardiogram (ECG) with wide complex tachycardia, one must differentiate between ventricular tachycardia and supraventricular tachycardia conducted with aberrancy. ECG criteria are used and algorithms developed to aid in differential diagnosis. They are based on finding ECG signs of ventriculoatrial dissociation and QRS morphologies inconsistent with classic bundle branch block. The conditions able to modify structurally the normal activation of the heart may alter spontaneous ventricular activation during supraventricular tachycardia, creating differential diagnosis problems.