  Key experimental results on unveiling and studying properties of a multiphase
state that arises in two-dimensional electron systems due to the interplay of
interelectron interactions and disorder are reviewed. The review focuses on the
experimental results obtained with high mobility Si-field effects structures
(Si-MOS), in which the interaction effects at low carrier concentrations are
most pronounced due to the strong e-e interactions, multi-valley spectrum, and
the short-range character of the random potential. The reviewed effects of
phase separation include features in transport, magnetotransport and
thermodynamics. Consideration of a number of experimental results is
supplemented with a brief review of their theoretical interpretation.
