Critical illness in patients with pre-existing diabetes frequently causes deterioration in glycaemic control.Despite the prevalence of diabetes in patients admitted to hospital and intensive care units,the ideal management of hyperglycaemia in these groups is uncertain.There are data that suggest that acute hyperglycaemia in critically ill patients without diabetes is associated with increased mortality and morbidity.Exogenous insulin to keep blood glucose concentrations < 10 mmol/L is accepted as standard of care in this group.However,preliminary data have recently been reported that suggest that chronic hyperglycaemia may result in conditioning,which protects these patients against damage mediated by acute hyperglycaemia.Furthermore,acute glucose-lowering to < 10 mmol/L in patients with diabetes with inadequate glycaemic control prior to their critical illness appears to have the capacity to cause harm.This review focuses on glycaemic control in critically ill patients with type 2 diabetes,the potential for harm from glucose-lowering and the rationale for personalised therapy. Critical illness in patients with pre-existing diabetes frequently causes deterioration in glycaemic control. Despite the prevalence of diabetes in patients admitted to hospital and intensive care units, the ideal management of hyperglycaemia in these groups is Uncertain. Here are data that suggest that acute hyperglycaemia in critically ill patients without diabetes is associated with increased mortality and morbidity. Exogenous insulin to keep blood glucose concentrations <10 mmol / L is accepted as standard of care in this group. Still, preliminary data have recently been reported that suggest that chronic hyperglycaemia may result in conditioning, which protects these patients against damage mediated by acute hyperglycaemia. Frthermore, acute glucose-lowering to <10 mmol / L in patients with diabetes with inadequate glycaemic control prior to their critical illness appears to have the capacity to cause harm. review focuses on glycaemic control in critically ill patients with type 2 di abetes, the potential for harm from glucose-lowering and the rationale for personalized therapy.