Insulin resistance has a major role in the development of impaired glucose tolerance and diabetes. Type 1 Diabetes mellitus (DM) is characterized by the destruction of P-cells and insulin deficiency. However, the presence of insulin resistance was shown in patients with Type 1 DM. Insulin resistance was first demonstrated with the insulin clamp technique in Type 1 DM in 1982. Later, other studies confirmed that the patients with Type 1 DM had marked insulin resistance compared to control groups. Chronic hyperglycemia is suggested to be mainly responsible for insulin resistance developing in Type 1 DM. Experiments in rats showed that the hexosamine pathway increased and insulin receptor substrate 1 (IRS 1) decreased. In addition, changes in kinetics of insulin-insulin receptor interaction are responsible for insulin resistance. Blood circulation of skeletal muscle and glucose uptake seemed to decrease in patients with poor glycemic control in addition to the presence of hepatic insulin resistance. Moreover, increased cardiovascular risk factors and waist-hip circumference were reported to be determinants of insulin resistance. Increased GH and decreased insulin like growth factor-1 (IGF-1) levels in puberty may also lead to a decrease in insulin sensitivity. Some studies report that metformin or glitazones may be effective in the treatment. In conclusion, the diagnosis of insulin resistance may have a positive effect on the regulation of diabetes. Especially, the treatment of insulin resistance may be beneficial for Type 1 diabetic patients with a body mass index > 30, increased need of insulin, hypertension and hypercholesterolemia.