The interplay between strong electron-electron interactions and disorder has been a long-standing question in condensed matter physics. The discovery of the metal-insulator transition (MIT) in two-dimensional (2D) electron systems challenged one of the most influential theories of the last century which stated that "there is no true metallic state in 2D". However, the scaling theory of localization did not account for interactions between electrons. This book provides recent thermodynamic and transport experimental findings which indicate that MIT in 2D can be considered as a quantum phase...