Control of glucose levels in diabetics is difficult during illness and perioperatively; they are affected by counter-regulatory hormones, unpredictable eating, lack of exercise and many other factors. Tight control is important: it improves outcomes in critically ill patients and reduces infection rates. A variety of different methods have emerged to administer IV insulin, inculding fixed and variable rate insulin infusions. This chapter provides a tutorial on variable rate insulin infusions, including a worked example, what monitoring is required and possible pitfalls.