Improvements in accuracy of real-time continuous glucose monitoring facilitate the development of closed-loop systems consisting of a continuous glucose monitor, a control algorithm, and an insulin pump. Closed-loop systems can be divided according to the way meal delivery is handled as "fully closed-loop" or "closed-loop" with meal announcement systems. Depending on the subcutaneous (sc) or intravenous (iv) body interface, three major types of closed-loop systems are recognised, (i) sc sensing and sc delivery system, (ii) the iv sensing and intraperitoneal delivery system, and (iii) the iv glucose sensing and iv insulin delivery system. Given the current research focus, this review centres on the sc-sc closed-loop approach, which has the greatest potential for a near-future commercial exploitation as recognised by the JDRF Artificial Pancreas Project. Other approaches utilising intraperitoneal or intravenous sensing/delivery are also discussed. Closed-loop systems may revolutionise diabetes management but their introduction is likely to be gradual starting from simpler applications such as hypoglycaemia prevention or overnight glucose control progressing to more complex approaches such as 24/7 glucose control. The most important question is what is achievable with existing technologies and when the first generation of closed-loop systems will find its way into clinical practice.