  We study the competition between pinning of a charge density wave (CDW) by
random distributed impurities and a periodic potential of the underlying
crystal lattice. In d=3 dimensions, we find for commensurate phases of order
p>p_c\approx 6/\pi a disorder driven continuous roughening transition from a
'flat' phase with translational long range order to a 'rough' glassy phase with
quasi long range order. Critical exponents are calculated in a double expansion
in \mu=p^2/p_c^2-1 and \epsilon=4-d and fulfill the scaling relations of random
field models. Implications for flux line lattices in high temperature
superconductors are briefly discussed.
