  In this overview we provide a general introduction to metal-insulator
transitions, with focus on specific mechanisms that can localize the electrons
in absence of magnetic or charge ordering, and produce well defined quantum
critical behavior. We contrast the physical picture of Mott, who emphasized the
role of electron-electron interactions, and that of Anderson, who stressed the
possibility of impurity-induced bound state formation, as alternative routes to
arrest the electronic motion. We also describe more complicated situations when
both phenomena play coexist, leading to meta-stability, slow relaxation, and
glassy behavior of electrons. A critical overview of the available theoretical
approaches is then presented, contrasting the weak-coupling perspective, which
emphasizes diffusion-mode corrections, and the strong-coupling viewpoint, which
stresses inhomogeneous phases and local correlation effects. We give specific
examples of experimental systems, providing clues on what should be the most
profitable path forward in unraveling the mystery of metal-insulator
transitions.
