Glucose monitoring systems emerged as an option to measure glucose concentrations in the interstitial fluid using a sensor. Main findings of this thesis are: • To achieve a glucose measurement with the smallest measurement uncertainty when diagnosing (gestational) diabetes, measurements should be performed using a FC-Mix blood collection tube instead of the standard NaF-EDTA or NaF-oxalate blood collection tube. • When using the FreeStyle Libre Flash Glucose Monitoring System (FSL-FGM) in daily life, it should be taken in account that readings in the lower range are likely to be lower than the actual glucose reading. Furthermore, after ingestion of a meal, FSL-FGM measured glucose lag behind the blood measured glucose. As a result, there is a risk of underestimating the effect of a meal on the glucose response • One-year use of FSL-FGM in people with diabetes resulted in improved well-being and decreased disease burden, as well as improvement of glycemic control, especially at those users with initially the poorest metabolic control. From a user perspective, the beneficial influence on the frequency and severity of hypoglycemia’s was also of great importance. • In a considerable percentage of people with diabetes, HbA1c levels reported by themselves are not accurate enough to be useful in research. • In case the glucose outcome derived from a glucose monitoring system does not match with the expected outcome by the user, a capillary glucose measurement should be performed. This is of particular importance during exercise.