Diabetes is a chronic disease with a high prevalence, which goes increasing gradually until getting features of epidemic. Since 芦Diabetes Control and Complications Trial禄 (DCCT) and 芦United Kingdon Prospective Diabetes Study禄 (UKPDS) were released in the 90's, Glucohemoglobin (HbA1c) has become the glycemic control imagine. This fact is responsible for its use as fundamental piece in the journey of the care of the patients with Diabetes. In spite of its importance and its applications, HbA1c is not generally known. In this work we expect to give the knowledge of many aspects of HbA1c from its definition to its diary clinic application. In this first part, we will revise the different points of the 芦Diabetes complex禄 where HbA1c is used and we will know its chemical definition.