The conduction system beyond the atrioventricular (AV) node normally runs in the interventricular septum. The normal route for electrical depolarization is through the AV node into the bundle of His, which bifurcates into the right bundle branch and the left bundle branch. The right bundle branch does not divide again, but the left bundle branch does divide into the left anterior fascicle and the left posterior fascicle. Thus, there are three identifiable conduction paths at the end portion of the ventricular bundle system: (1) the right bundle branch, (2) the left anterior fascicle, and (3) the left posterior fascicle. Any one or combination of these fascicles may have an abnormal delay in conduction, causing a bundle branch block, a hemiblock, or even trifascicular block. This chapter describes the typical electrocardiographic features associated with each of these conduction disturbances.