Fetal arrhythmias are seen in about two per cent of pregnancies. In many cases, arrhythmias are found in the office on routine auscultation of fetal heart tones by midwives, nurses, and obstetricians, and at home by mothers using readily available doptone devices. On real-time greyscale ultrasound, significant bradyarrhythmias and tachyarrhythmias can be discerned, and an irregular cardiac rhythm can also be subjectively appreciated. A more nuanced diagnosis requires additional evaluation tools. Postnatal arrhythmia can be readily diagnosed with an ECG, but clinically adequate fetal ECG is not currently possible. Fetal magnetography, which gives results similar to ECG, is available only in a few specialised high-resource settings. For practical purposes, echocardiography is the only, and currently best tool for diagnosing and monitoring significant fetal arrhythmia. M-mode, spectral Doppler, and tissue Doppler are modalities commonly used to diagnose fetal arrhythmia. This review addresses both M-mode and Doppler spectra, but Doppler diagnosis is discussed in more detail. Basic concepts of electrophysiology and pathophysiology, and descriptions of normal and abnormal Doppler spectra are introduced, and the diagnostic algorithm is built on this foundation.