||Mitochondria are complex organelles that contribute to a wide range of cellular functions, including energy production, biosynthetic pathways, lipid metabolism, heme synthesis, and programmed cell death. Consistent with these multiple activities, mitochondrial dysfunction plays a central role in many diseases, including cancer, neurodegeneration, diabetes, and inborn errors of metabolism. As a result, many studies are currently focused on defining the roles of mitochondrial proteins in cellular function and organismal physiology as well as how mitochondrial dysfunction can lead to disease. My dissertation research focused on characterizing two evolutionarily conserved mitochondrial proteins using Drosophila as a model system: the Aarf Domain Containing Kinase 1 (ADCK1) and the Mitochondrial Pyruvate Carrier (MPC). Studies of the ADCK family of predicted mitochondrial kinases have centered on the Abc1/Coq8 subfamily in yeast and humans, which is involved in Coenzyme Q biosynthesis. In contrast, little is known about ADCK1, which is thought to play a role in intracellular lipid trafficking in yeast. We show that Drosophila ADCK1 mutants die as second instar larvae with double mouthhooks and tracheal breaks, defects associated with reduced levels of the steroid hormone ecdysone. Mutants for C. elegans ADCK1 result in a reduced brood size, which is enhanced by depriving the mothers of cholesterol. These mutants also display elevated levels of yolk proteins, which mediate cholesterol uptake and lipid transport. Our developmental genetic studies thus support a model in which ADCK1 is required in flies and worms for proper lipid trafficking and/or availability, and provide a physiological context for future studies of ADCK1 function. The MPC is required for efficient mitochondrial pyruvate uptake, linking cytoplasmic glycolysis with mitochondrial pyruvate oxidation. This placement of the MPC allows for precise genetic manipulation of mitochondrial pyruvate metabolism in a cellspecific manner. My research exploited this opportunity through studies in Drosophila insulin producing cells (IPCs), stem cells, cancer stem cells, and nonmitotic larval fat cells. Loss of MPC function in Drosophila IPCs leads to defects in glucose stimulated insulin secretion that manifest as hallmarks of diabetes, including glucose intolerance and hyperglycemia. In contrast, genetic studies of the MPC in intestinal stem cells showed that mitochondrial pyruvate uptake and metabolism is necessary and sufficient to regulate stem cell proliferation and maintain intestinal homeostasis. Moreover, mitochondrial pyruvate metabolism plays an active direct role in regulating cancer stem cell proliferation in two different tumor models. Unexpectedly, we also discovered that cell size could be reduced by enhancing mitochondrial pyruvate metabolism in a glycolytic postmitotic cell type, Drosophila larval fat cells. Overall, my dissertation research exemplifies how changes in mitochondrial metabolism can play an active and direct role in cellular function, cell proliferation, tumor formation, and animal physiology. Additionally, I report preliminary evidence that ADCK1 regulates lipid homeostasis during development.