  By using two ab initio numerical methods we study the effects that disorder
has on the spectral gaps and on wave localization in two-dimensional photonic
band gap materials. We find that there are basically two different responses
depending on the lattice realization (solid dielectric cylinders in air or vise
versa), the wave polarization, and the particular form under which disorder is
introduced. Two different pictures for the photonic states are employed, the
``nearly free'' photon and the ``strongly localized'' photon. These originate
from the two different mechanisms responsible for the formation of the spectral
gaps, ie. multiple scattering and single scatterer resonances, and they
qualitatively explain our results.
