Since type I atrioventricular (A-V) block tends to be temporary and does not give rise to prolonged asystole, and type II A-V block tends to be progressive, ultimately leading to complete block and Adams-Stokes attacks, it is important to identify the type in each patient with acute or chronic second degree A-V block. The usual definition of these 2 types needs expansion. This new definition is given and new criteria are presented to permit differentiation of the 2 types of A-V block even in the presence of (1) incomplete A-V dissociation with single ventricular captures and (2) persistent 2:1 A-V block. The different roles and sites of concealed conduction in the 2 types of A-V block are defined.In type I, concealed conduction occurs within the region of block and depresses subsequent conduction, occasionally leading to blockage of consecutive atrial impulses. In type II, concealed conduction occurs down to the region of block, discharging subsidiary junctional pacemakers and thus preventing their escape. The frequent association of type II A-V block with bundle branch block and ventricular escapes with normal retrograde conduction (unidirectional block) is emphasized. Concealed retrograde conduction across the region of unidirectional block facilitates antegrade A-V transmission (early ventricular captures, supernormal phase of A-V conduction). The evidence is reviewed for placing the lesion causing type II A-V block below the A-V node; this is based on correlation of electrocardiographic findings, including His bundle recordings and anatomic data. Exceptions to the rule occur occasionally but should not lead to abandonment of the electrocardiographic classification of second degree A-V block into type I and type II because the distinction has proved of great value for clinical orientation.