Insulin Sensitivity, Impaired Counterregulation, and Glucose Variability

Inclusion Criteria: * Type I diabetes mellitus (TIDM) (as defined by the American Diabetes Association criteria or judgment of a physician) for at least two years prior to the enrollment in the study. * Use of an insulin pump to treat their diabetes for at least six months prior to the study. * Ability to use a bolus calculator function with the current insulin pump with pre-defined parameters for glucose goal, carbohydrate ratio, and insulin sensitivity factor. * Age 21 - 64 years. Adults age 65 and over are likely to have medical exclusions for the follow-up Phase 2 study, which involves induced hypoglycemia. * Completed Phase 1 of the Institutional Review Board (IRB) #14956 "Development of a Behavioral Observer for Type 1 Diabetes Mellitus" * Willingness to use lispro (Humalog) insulin two days prior to and during inpatient admission. * Willingness to perform Self-Monitoring Blood Glucose (SMBG) 4 times per day at times before meals and bedtime. * Demonstration of proper mental status and cognition for completion of the study. Exclusion Criteria: * Pregnancy * Psychiatric disorders that would interfere with study tasks (e.g. mental retardation, substance abuse) * History of a systemic deep tissue infection with methicillin-resistant staph aureus or Candida albicans * Known bleeding diathesis or dyscrasia * Active enrollment in another clinical trial * Medical condition that would make operating an insulin pump difficult (e.g. blindness, severe arthritis, extensive scar tissue at sites where devices are inserted). * Anemia (hematocrit \<36% \[females\], \<38% \[males\]), * Allergy or adverse reaction to lispro (Humalog) insulin * Conditions which may increase the risk of induced hypoglycemia such as symptomatic heart disease, congestive heart failure, history of a cerebrovascular event, atrial fibrillation, renal insufficiency (creatinine \>1.5) or uncontrolled hypertension (resting blood pressure \>140/90).