  This review first describes the evidence that strongly suggests the existence
of the metal-insulator transition (MIT) in a two-dimensional electron system in
Si regardless of the amount of disorder. Extensive studies of the charge
dynamics demonstrate that this transition is closely related to the glassy
freezing of electrons as temperature T->0. Similarities to the behavior of
three-dimensional materials raise the intriguing possibility that such
correlated dynamics might be a universal feature of the MIT regardless of the
dimensionality.
