Improving mean levels of glycemic control as judged by assays for HbA1c has been the primary target to reduce the risks of microvascular complications and cardiovascular events in patients with type 1 and type 2 diabetes. However, observations in some clinical trials including ACCORD and DCCT have suggested that lowering to suggested targets can increase severe hypoglycemia and mortality in some patients with diabetes who have high level of GV. Therefore, lowering GV could be an important target to achieve targeted A1C without hypoglycemia in these patients. GV can be measured more precisely by continuous glucose monitoring (CGM) compared to self monitoring of blood glucose (SMBG). CGM has been commercially available since the mid 2000s, and this technology is evolving steadily in terms of accuracy and ease of use. Recent studies showed that real-time CGM and flash glucose monitoring (FGM) improve glycemic control and reduces the risk of hypoglycemia in patients with type 1 diabetes and type 2 diabetes with intensive insulin treatment, and it is related with lowering GV by real-time CGM. Short-term, intermittent use of retrospective CGM has been shown to be effective to reduce HbA1c in patients with type 2 diabetes (not on prandial insulin) with HbA1c of 7% or greater. New technology such as newer insulin, CGM, sensor augmented pump, and artificial pancreas can reduce A1C and hypoglycemia by decreasing GV in patients with type 1 diabetes. Concomitant reduction of mean glucose and glycemic variability would be achieved by newer glucose lowering agents and more routine continuous glucose monitoring for both type 1 and type 2 diabetes. In this lecture, I will review recent studies for GV and CGM and discuss the clinical importance of GV in CGM.