Global diabetes epidemics is currently representing one of the most prominent medical and societal challenges. Hemoglobin A1c (HbA1c), a biochemical marker of an average blood glucose concentration has been used for more than 30 years as a clinical indicator of both diabetes treatment efficacy and the risk for development of complications. Recently, HbA1C was proposed as a diabetes diagnostic test as well. Regular monitoring of glycemic control and adjustment of therapy towards the recommended HbA1c-based treatment-goals is a pivotal request of contemporary diabetes care guidelines, as well as a quality indicator proclaimed by numerous national health-care-delivery systems. Standardized and attainable analytical methodology of high-quality and a good knowledge on determinants of biological variability, able to influence test results, are crucial elements for the confident clinical use of HbA1c. In this review, essential analytical and clinical aspects necessary for the reliable use of HbA1c results in diabetes care are concisely presented, together with the degree of Croatian laboratory and clinical practice harmonization with the relevant international standards.