Glycaemia can be measured by numerous methods, the main ones being reduction methods--which measure both glucose and other reducing agents in the blood and are still widely used after some fifty years--and enzymatic methods, which measure only blood glucose and, consequently, give lower values. Attempts have recently be made at increasing the selectivity of reduction methods, the values obtained being comprised between these two sets of figures. All this may confuse the physician, who should know that within limit values normal and pathological results can only be differentiated by taking into account the assay method. In the future, measurement of glycosyl haemoglobin will probably be used to complement blood glucose assays.