  We present an overview of the measured transport properties of the two
dimensional electron fluids in high mobility semiconductor devices with low
electron densities, and of some of the theories that have been proposed to
account for them. Many features of the observations are not easily reconciled
with a description based on the well understood physics of weakly interacting
quasiparticles in a disordered medium. Rather, they reflect new physics
associated with strong correlation effects, which warrant further study.
