Glycated haemoglobins are haemoglobins with an attached sugar moiety. They constitute the HbA1 fraction of the adult haemoglobin HbA. HbA1c is the predominant fraction of HbA1 and gives an estimate of the blood sugar levels of an individual over the last three months. It has been observed that an HbA1c value of less than seven percent reduces the microvascular complications in diabetic patients. However, HbA1c is not affected by blood sugar levels alone. Apart from blood sugar, there are other factors that affect HbA1c. This article reviews in detail the structure, formation, methods of measurement, factors affecting HbA1c levels and their clinical significance.