This is an open-label, single-center, randomized crossover study evaluating the performance of a closed-loop insulin delivery system using a subcutaneous glucose sensor and an external insulin pump and comparing it to an open-loop delivery system during aerobic exercise. Subject participation will be for a maximum of 16 weeks including an outpatient study (up to 6 days) and two inpatient studies (one closed-loop; the other open-loop). The closed- and open-loop visit consists of approximately 67 hours over 4 days and 3 nights.