  Two-dimensional electrons in AlAs quantum wells occupy multiple
conduction-band minima at the X- points of the Brillouin zone. These valleys
have large effective mass and g-factor compared to the stan-dard GaAs
electrons, and are also highly anisotropic. With proper choice of well width
and by applying symmetry-breaking strain in the plane, one can control the
occupation of different valleys thus rendering a system with tuneable effective
mass, g-factor, Fermi contour anisotropy, and valley degeneracy. Here we review
some of the rich physics that this system has allowed us to explore.
