Impurities which are distributed at random in the crystal have a profound influence on both the static and dynamic properties of density waves. The length scale depends on the strength of the impurity potentials, on the impurity concentration, and on the properties of the density waves. The various situations which may occur can easily be seen for a one-dimensional density wave interacting with randomly positioned impurities. The disorder and the finite coherence length also lead to a distribution of energy levels for the various pinned density wave configurations, with thermally induced transitions between those configurations. A more significant consequence of impurity-density wave interactions is the pinning of the collective modes to the underlying lattice. Due to this interaction charge and spin density waves cannot move freely in the lattice; but assume a well defined equilibrium position which is determined by the spatial distribution of impurities.