  We experimentally investigate the evolution of linear and nonlinear waves in
a realization of the Anderson model using disordered one dimensional waveguide
lattices. Two types of localized eigenmodes, flat-phased and staggered, are
directly measured. Nonlinear perturbations enhances localization in one type,
and induce delocalization in the other. In a complementary approach, we study
the evolution on short time scales of $\delta$-like wavepackets in the presence
of disorder. A transition from ballistic wavepacket expansion to exponential
(Anderson) localization is observed. We find an intermediate regime in which
the ballistic and localized components coexist while diffusive dynamics is
absent. Evidence is found for a faster transition into localization under
nonlinear conditions.
