  Experimental evidence for the possible universality classes of the
metal-insulator transition (MIT) in two dimensions (2D) is discussed.
Sufficiently strong disorder, in particular, changes the nature of the
transition. Comprehensive studies of the charge dynamics are also reviewed,
describing evidence that the MIT in a 2D electron system in silicon should be
viewed as the melting of the Coulomb glass. Comparisons are made to recent
results on novel 2D materials and quasi-2D strongly correlated systems, such as
cuprates.
