Book excerpt: Obesity and type 2 diabetes are modern epidemics, closely associated with the development of insulin resistance. While lipid induced mitochondrial dysfunction plays an important role in skeletal muscle insulin resistance, it is unclear whether a similar etiology is responsible for hepatic insulin resistance. The purpose of this thesis is to investigate whether impaired mitochondrial function plays a role in hepatic insulin resistance, and if so, whether it is a cause or consequence of insulin resistance. I further put forth the hypothesis that increased fat delivery either by diet or by genetic defect results in elevated hepatic triglyceride storage, induction of hepatic TCA cycle flux and poorly controlled gluconeogenesis in the insulin resistant liver. In order determine in vivo hepatic metabolism, we first developed a stable isotope tracer method to estimate hepatic fat oxidation and integrated it with other standard techniques to measure hepatic glucose production. These methods were used to determine in vivo hepatic mitochondrial pathways in a diet induced obese and diabetic mouse model and in a severe genetic model of diabetes (ZDF rats). Additionally, a pharmacological intervention in ZDF rats was used as an attempt to correct the dysfunctional metabolic pathways. The findings from this study suggest that mitochondrial dysfunction is not a cause, but the consequence of hepatic insulin resistance. The data further indicate that only selected mitochondrial pathways are impaired during severe insulin resistance, while others are induced and that interventions that target mitochondrial metabolism may not completely normalize hepatic metabolism if they indiscriminately induce all mitochondrial pathways.