Hyperglycemia is commonly observed in critical illness. A landmark large randomized controlled trial (RCT) reported that the incidence of hyperglycemia (blood glucose concentration > 108 mg/dl) was as high as 97.2% in critically ill patients. During the past two decades, a number of RCTs and several meta-analyses and network meta-analyses have been conducted to determine the optimal target for acute glycemic control. The results of those studies suggest that serum glucose concentration would be better to be maintained between 144 and 180 mg/dl. Although there have been studies showing an association of hypoglycemia with worsened clinical outcomes, a causal link has yet to be confirmed. Nonetheless, some researchers are of the view that the data suggest even mild hypoglycemia should be avoided in critically ill patients. Since acutely ill patients who receive insulin infusion are at a higher risk of hypoglycemia, a reliable devices for measuring blood glucose concentrations, such as an arterial blood gas analyzer, should be used frequently. Acute glycemic control in patients with premorbid hyperglycemia is a novel issue. Available literature suggests that blood glucose concentrations considered to be desirable and/or safe in non-diabetic critically ill patients might not be desirable in patients with diabetes. Moreover, the optimal target for acute blood glucose control may be higher in critically ill patients with premorbid hyperglycemia. Further study is required to assess optimal blood glucose control in acutely ill patients with premorbid hyperglycemia.