Elevated levels of glucose and/or lipids in Type 2 diabetes (T2DM) are thought to negatively affect islet function and survival (so-called `glucotoxicity', `lipotoxicity', or `gluco-lipotoxiciy'), leading to a further impairment of insulin secretion and worsening glycemic control. However, much of the evidence for glucotoxicity or lipotoxicity has come from studies of rodent islets or rodent models of T2DM (ob/ob, db/db, ZDF, high-fat diet, etc.). This is problematic because human islets have critical differences from rodent islets. Thus, our understanding of the mechanistic events in human islets exposed to excess glucose and lipid in vivo is quite limited. These fundamental gaps in our knowledge, which are partly the result of experimental limitations of studying human islets in vivo and partly from limitations in our knowledge of human islet biology, restrict our ability to develop interventions to preserve beta cell function. To address these limitations, we developed new pre-clinical, experimental approaches that allow mechanistic analysis of human islets exposed to excess glucose and/or lipid in vivo (human islets transplanted into immunodeficient mice). Based on our in vivo and in vitro data, we hypothesize that excess glucose and/or lipid in vivo (glucotoxicity or lipotoxicity) compromises the function and/or expression of these key islet-enriched transcription factors in human islets, leading to impaired insulin secretion and biosynthesis and the worsening islet function in T2DM. We propose three aims to test this hypothesis: 1) Define the degree, duration, and mechanism(s) by which hyperglycemia alters human islet function in vivo. 2) Determine whether excess lipid worsens the effects of hyperglycemia on human islet function in vivo. 3) Determine whether excess glucose and/or lipid impair human islet function in vivo by similar or distinct mechanisms. The proposed studies bring together new information and technologies related to islet-enriched transcription factors and human islet biology, as well as new experimental models that allow for human islets to be studied in vivo in ways that have not previously been possible. The proposed studies address a critical problem in T2DM, namely the molecular mechanisms of glucotoxicity and/or lipotoxicity, and should drive new efforts to design interventions to preserve or improve beta cell function.