The scalng theory of localization for non-interacting electrons predicts that all states in 2D are localized by any amount of disorder in the zero temperature limit. In contrast, a "metallic" behavior characterized by a decreasing resistivity with decreasing temperaure, and thus an apparent metal-insulator transition as the charge-carrier density is reduced, is observed on many different low disorder dilute 2D systems such as Si MOSFET, GaAs, SiGe, and AlAs at low temperatures. It is found that the charge-carrier density where a metal-insulator transition occurs monotonically decreases with decreasing disorder. In an exceptionally high mobility 2D holes in GaAs heterostructure, the metal-insulator transition is observed at a density where the clean quantum Wigner crystal is expected, suggesting that the metal-insulator transition is related to the melting of the Wigner crystal. Details of transport characteristics in this GaAs system will be presented.