An atrioventricular accessory pathway (AP) may be manifest or concealed. When manifest, it leads to preexcitation on the baseline electrocardiogram, which is called the Wolff-Parkinson-White pattern. The degree of preexcitation varies according to the relative conduction speed of the atrioventricular node versus the AP, the AP location, and the AP refractory period. This explains that even a manifest AP may lead to only intermittent preexcitation. The AP conducts faster than the atrioventricular node but has a longer refractory period, which allows the initiation of a reentrant arrhythmia called atrioventricular reciprocating tachycardia. In addition to re-entry, a manifest AP may allow the fast antegrade conduction of an atrial tachyarrhythmia, leading to a small risk of sudden death; the latter depends on the AP refractory period (ie, the number of atrial waves it can conduct back to back) rather than the AP conduction speed. This can be assessed invasively and noninvasively and allows risk stratification of asymptomatic individuals.