The objective of the proposed research is to evaluate the efficacy and safety of the use of metformin in addition to standard insulin therapy in overweight and obese children and adolescents, age 12-<20 years, with type 1 diabetes for at least 1 year. Secondary objectives are to assess the effect of metformin on C-peptide levels, a measure of how much insulin is still being produced by the beta cells of the pancreas, and on vascular dysfunction. In addition, an ancillary study is planned to assess if metformin will improve tissue-specific insulin resistance in type 1 diabetes using a hyperinsulinemic euglycemic clamp.