The gold standard for the assessment of the overall glycemic control is the determination of HbA1c. There are, however, insufficient data to determine reliably the relative contribution of fasting and postprandial plasma glucose to HbA1c. Increasing evidence suggests that excessive excursions of postprandial glucose might be important for the development of micro- and macroangiopathic complications. With respect to the treatment options, one important question to be answered is whether premeal, postmeal or fasting plasma glucose, alone or in combination, will be necessary in adjusting the therapy to achieve optimal HbA1c levels while minimizing hypoglycemia. HbA1c is difficult to predict from fasting plasma glucose. There are indications that there is a shift in the relative contribution from postprandial glucose at good to fair HbA1c levels (9.3%). There is also a better correlation of afternoon and evening plasma glucose with HbA1c than with prebreakfast and prelunch plasma glucose values. Since the definition on how to define postprandial glucose is still a matter of debate and since postprandial glucose depends on the premeal blood glucose level and, on the time of the meal, its size and composition and the therapeutic strategy, the data so far available are inconclusive and the best correlation of HbA1c is with the area under the glucose profiles. Continuous glucose monitoring under daily life conditions will be the key to definitely unravel the relationship among HbA1c and fasting, premeal, postprandial and postabsorptive plasma glucose.