Conduction slowing or block can occur in the atrioventricular (AV) node, the bundle of His, or both, and manifest as characteristic patterns in the electrocardiogram and intracardiac electrogram. These patterns lead to differences in prognosis and therapy, depending on the location and severity of disease. In general, the prognosis is better when the level of block is above the AV node (supra-His), whereas block below the AV node (intra-His or infra-His) portends a higher risk of progression to complete heart block. Causes of AV block are numerous and recognition of the cause is essential, because many are easily treated.