Stress hyperglycaemia, hypoglycaemia, and diabetes are common in critically ill patients and related to clinical endpoints. To avoid complications related to hypoglycaemia and hyperglycaemia, it is recommended to start insulin therapy for the majority of critically ill patients with persistent blood glucose concentrations higher than 10·0 mmol/L (>180 mg/dL), targeting a range of 7·8-10·0 mmol/L (140-180 mg/dL). However, management and evidence-based targets for blood glucose control are under debate, particularly for patients with diabetes. Recent randomised controlled clinical trials now challenge current recommendations. In this Personal View, we aim to highlight these developments and the important differences between critically ill patients with and without diabetes, taking into account the considerable heterogeneity in this patient group. We critically discuss evidence from prospective randomised controlled trials and observational studies on the safety and efficacy of glycaemic control, specifically in the context of patients with diabetes in intensive care units.