  We review recent and give some new results on the spectral properties of
Schroedinger operators with a random potential of alloy type. Our point of
interest is the so called Wegner estimate in the case where the single site
potentials change sign. The indefinitness of the single site potential poses
certain difficulties for the proof of the Wegner estimate which are still not
fully understood.
  The Wegner estimate is a key ingredient in an existence proof of pure point
spectrum of the considered random Schroedinger operators. Under certain
assumptions on the considered models additionally the existence of the density
of states can be proven.
