I will give a brief overview of the history of two arrhythmias in which the atrioventricular (AV) node is involved. 1. Atrioventricular reentrant tachycardia. After describing circulating excitation in ring-like preparations of hearts of a variety of species, G.R.Mines (1) wrote in 1913: “I venture to suggest that a circulating excitation of this type may be responsible for some cases of paroxysmal tachycardia as observed clinically”. One year later, he repeated this suggestion after reading Stanley Kent’s description of a connection between the right atrium and the right ventricle (2): “ Supposing that for some reason an impulse from the auricle reached the main A-V bundle but failed to reach this “right lateral” connection, it is possible then that the ventricle would excite the ventricular end of this right lateral connection, not finding it refractory as it normally would at such a time. The wave spreading up to the auricle might be expected to circulate around the path indicated “(3) This was written 16 years before Wolff, Parkinson and White described the clinical syndrome that now bears their name (4), 18 years before Holzmann and Scherf (5) ascribed the abnormal ECG in these patients to pre-excitation of the ventricles via an accessory AV bundle, 19 years before Wolferth and Wood (6) published the first diagrams showing the pathway for orthodromic and antidromic reentry, and 53 years before the first studies in patients employing intraoperative mapping and programmed stimulation during cardiac catheterization proved Mines, predictions to be correct (7). It is remarkable that none of these studies quoted Mines. The era of surgical ablation of the accessory pathway started in 1967, and was initially hampered by not realizing the correct anatomy of the accessory pathway, which was quite different from what Kent had described (2). In 1944 Öhnell showed that the accessory bundle did not penetrate the fibrous annulus, but coursed in the epicardial fat surrounding the coronary arteries. After Sealy and colleagues in 1976 developed a “fish hook” to scrape through the epicardial fat, surgical treatment became successful. It also paved the way for the hugely successful catheter ablation. 2. Atrioventricular nodal reentrant tachycardia. Mines (1) also was the first to describe AV nodal reentry, which he called a reciprocating rhythm. He postulated that the different fibres in the AV node “ are ordinarily in physiologic continuity, yet it is conceivable that exceptionally, as after too rapid stimulation, different parts of the bundle should lose their intimate connection… A slight difference in the rate of recovery of two divisions of the A-V connection might determine that an extrasystole of the ventricle, provoked by a stimulus applied to the ventricle shortly after activity of the A-V connection, should spread up to the auricle by that part of the A-V connection having the quicker recovery process and not by the other part. In such a case, when the auricle became excited by this impulse, the other portion of the A-V connection would be ready to take up transmission again back to the ventricle. …the condition once established would tend to continue, unless upset by the interpolation of a premature systole” (1). It took more than half a century before upsetting AV nodal reentry by “premature systoles’ was accomplished in patients and in isolated rabbit heart preparations. Both papers did quote Mines. Although all authors working on AV nodal reentry agree that that the lower level of the junction of antegrade and retrograde pathways is above the level of the His bundle, controversy has existed regarding the question whether or not the atrium forms part of the reentrant circuit. The fact that it is possible, both by surgery and catheter ablation, to abolish AV nodal reentry by destroying tissue far away from the compact AV node whilst preserving AV conduction seems clear evidence that the atrium must be involved. However, in the canine heart the reentrant circuit during atrial and ventricular echo beats is confined to to the compact node and regions immediately adjacent to it, and atrial tissue is not involved. To quote Zipes, who borrowed the words that Churchill used to characterize Russia : “The AV node is a riddle wrapped in a mystery inside an enigma”.