Department of PhysicsTransitions between metallic and insulating behavior occur in many materials as some parameter of the system is varied. Examples of metal-insulator transitions in bulk, three-dimensional semiconductors will be shown. Based on both theory and experiment, however, it was believed for many years that no metallic phase is possible in two dimensions (systems that exists on a plane or a surface). It was therefore a surprise and a source of great controversy when behavior characteristic of a metal was observed. Is there a true metallic phase in two dimensions? Do these observations signal exciting new physics or can they be explained by extending known physics into a new regime? A brief history will be presented and the current situation will be discussed. New results will be shown if time permits. <Slides from the Presentation>
