Wolff-Parkinson-White syndrome is a cardiac conduction disorder characterised by an accessory electrical pathway between the atria and ventricles, causing pre-excitation of the ventricles and a distinctive delta wave on the ECG. It is clinically important because it predisposes patients to episodes of supraventricular tachycardia and increases the risk of sudden cardiac arrest. A key complication is atrial fibrillation conducting rapidly to the ventricles via the accessory pathway, potentially leading to ventricular fibrillation.