Determining the level of glycated haemoglobin, in particular its major fraction called HbA(1c), is an attractive tool in the management of diabetic patients. In fact, it provides a global evaluation of the glycemic control's level through the past 8-12 weeks. However, this tool must be used with caution. First of all, it does not allow to examine the glycemic kinetics since it represents a glycemic average. Secondly, it does not allow to appreciate the glycemic evolution through the full day. This dosage needs then sometimes to be complemented by fingersticks blood glucose testing. Last but not least, caution is advised in interpreting the results because a number of physiological, pathological and technical factors might interfere with HbA(1c) measurement. It is therefore important that physicians keep a critical view of the values obtained. The paper reviews the different methods used to determine the level of glycated haemoglobin and their limitations. It also emphasizes the medical situations in which over- and under-estimation of the real HbA(1c) value could occur. It does not address the specific issue of the new expression values of HbA(1c) in mmol/mol instead of %. Moreover, the medical situations in which over- and underestimation of the real HbA(1c) value could occur will be described.