Type 1 diabetes (T1D) is often first recognized when signs and symptoms occur, yet the patho-genetic development of T1D usually begins years before that. Pancreatic autoantibodies commonly become elevated long before diagnosis (1). Although data indicate that the first-phase insulin response (FPIR) is also abnormal well before diagnosis (2–5), the development and progression of metabolic abnormalities had not been well characterized until the recent performance of T1D prevention trials (6–8). The unique designs of these trials provided the opportunity to perform longitudinal studies that have yielded new insights into metabolic changes that occur during the progression to T1D. This review will describe what we have learned about the metabolic natural history of T1D from the Diabetes Prevention Trial–Type 1 (DPT-1).