Insulin regulates the storage of dietary glucose by stimulating its uptake into muscle and fat. Insulin increases glucose uptake into these cells by recruiting vesicles containing the GLUT4 glucose transporter to the cell surface. Thus, insulin controls glucose uptake by regulating GLUT4 trafficking between the interior and cell surface. Understanding how insulin regulates GLUT4 traffic is key for understanding the molecular changes underlying type 2 diabetes. We use quantitative optical microscopy to study insulin-regulated membrane trafficking. The main objectives of our work are to characterize the GLUT4 trafficking pathway in the presence and absence of insulin, and to identify how the insulin-signal transduction regulates the movement of GLUT4 vesicles. In addition to studies of GLUT4 trafficking, we are also interested in more basic questions of membrane trafficking, specifically a more detailed understanding of the molecular mechanisms of clathrin-mediated internalization from the cell surface and the mechanisms for return of endocytosed proteins back to the plasma membrane.