Matter wave diffraction at periodic potentials made of light was first demonstrated in 1983 [1]. In Bragg diffraction, which is well known from X-ray scattering at solids, the interaction with a periodic potential is only weak and clearly shows wave properties. This effect was first observed with atoms at standing light waves in 1988 [2] and later studied in detail [3]. By increasing the light intensity one can realize higher potentials and can thus reach the regime of channeling, where now the atoms behave like particles and their propagation can be described with classical trajectories.