The electrocardiogram (ECG) remains the most commonly used test in medical practice and as such requires to be interpreted with due care and attention to detail. The ECG changes rapidly from birth through childhood with age differences clearly related to increasing QRS voltages and a widening QRS complex. The only sex difference at this age is a slightly longer QRS duration in boys than girls.In adulthood, sex differences in QRS voltage are maximum in the under 40 age group and tend to minimise with advancing age. QRS duration is longer in males than in females, but little difference is made of this in diagnostic criteria. In a similar vein, ST amplitudes are higher in young males compared to young females with the difference diminishing as age increases. Corrected QT interval is longer in females than males.In summary, age and gender differences in the ECG are important and have been incorporated into a variety of criteria for ECG interpretation. Physicians should be aware of the main sex differences in the ECG.