Mahaim and Winston were the first to describe Mahaim fiber, as a tract, which connects the atrioventricular (AV) node to the ventricular myocardium. Mahaim fiber conducts slowly and only antegradely and has decremental conduction properties, and these features represent the Mahaim physiology. Atriofascicular bypass tracts are the most common Mahaim fibers and are responsible for antidromic tachycardia that presents with left bundle branch block (LBBB) morphology and left axis deviation of the QRS in the frontal plane. Palpitation is the most common presentation and is related to either an antidromic atrioventricular reciprocating tachycardia (AVRT) or other supraventricular tachycardias, wherein the atriofascicular bypass tract acts as an innocent bystander. Atrioventricular bypass tract, nodofascicular/nodoventricular bypass tract and fasciculoventricular bypass tract are the other variants of preexcitation with Mahaim physiology. 22 figures are also provided in this chapter.