  Recent magneto-transport experiments on ultra-high mobility 2D electron
systems in GaAs/AlGaAs heterostructures have revealed the existence of whole
new classes of correlated many-electron states in highly excited Landau levels.
These new states, which appear only at extremely low temperatures, are
distinctly different from the familiar fractional quantum Hall liquids of the
lowest Landau level. Prominent among the recent findings are the discoveries of
giant anisotropies in the resistivity near half filling of the third and higher
Landau levels and the observation of re- entrant integer quantum Hall states in
the flanks of these same levels. This contribution will survey the present
status of this emerging field.
