Sustained bradyarrhythmias are typically the result of symptomatic sinus bradycardia, atrial bigeminy or complete atrioventricular (AV) block. Fetal tachyarrhythmias relate to sinus tachycardia, atrial flutter and supraventricular tachycardia as the main aetiology. Ultrasound is essential to understand the underlying arrhythmia mechanism, to study the impact on cardiac function, to exclude cardiac defects or tumours, and to survey the fetal heart rate and well-being, e.g. during anti-arrhythmic treatment. Based on retrospective studies, several more or less safe, effective and well-tolerated anti-arrhythmic agents are currently available for the treatment of atrial and supraventricular tachycardia. Isolated congenital complete AV block is mainly related to maternal anti-Ro/La auto-antibodies. The rationale to treat a fetus at this irreversible stage of AV nodal damage is primarily to mitigate or prevent concomitant myocardial inflammation and to augment cardiac output. A recently published study demonstrated a significant improved outcome with transmaternal dexamethasone and beta-stimulation.