The article reviews the significance of glycated hemoglobin (HbA1c) in the diagnosis and prognosis of diabetic patients. HbA1c is an important indicator of long-term glycemic control, reflecting the cumulative glycemic history over the preceding two to three months. It provides a reliable measure of chronic hyperglycemia and correlates well with the risk of long-term diabetes complications. Elevated HbA1c levels are also associated with an increased risk of coronary heart disease and stroke, both in diabetic and non-diabetic individuals. The review highlights the diagnostic and prognostic potential of HbA1c, its analytical aspects, and the physiological changes due to hemoglobin glycation. HbA1c is recommended as a standard test for monitoring and chronic management of diabetes, and its use can help in early detection and management of the disease, leading to better patient outcomes. The article also discusses the units and range of HbA1c, methods for its analysis, and the accessibility of HbA1c testing in different settings.The article reviews the significance of glycated hemoglobin (HbA1c) in the diagnosis and prognosis of diabetic patients. HbA1c is an important indicator of long-term glycemic control, reflecting the cumulative glycemic history over the preceding two to three months. It provides a reliable measure of chronic hyperglycemia and correlates well with the risk of long-term diabetes complications. Elevated HbA1c levels are also associated with an increased risk of coronary heart disease and stroke, both in diabetic and non-diabetic individuals. The review highlights the diagnostic and prognostic potential of HbA1c, its analytical aspects, and the physiological changes due to hemoglobin glycation. HbA1c is recommended as a standard test for monitoring and chronic management of diabetes, and its use can help in early detection and management of the disease, leading to better patient outcomes. The article also discusses the units and range of HbA1c, methods for its analysis, and the accessibility of HbA1c testing in different settings.