Atrial fibrillation is the commonest cardiac arrhythmia encountered in clinical practice. In this condition, chaotic electrical impulses, generated from multiple sites within the atria and pulmonary veins, result in irregular depolarisation of the ventricles with a resulting irregularly irregular heartbeat. Recognition of atrial fibrillation on the ECG is a crucial skill as the arrhythmia increases the risk of stroke and heart failure. These complications are preventable with appropriate treatment. Atrial flutter is a common arrhythmia which arises by a very specific mechanism. This arrhythmia is easily missed on the ECG.