Light propagation in multimode photonic crystal (PC) waveguides and in arrays of single-mode PC waveguides is analyzed. The dispersion relations of the modes traveling along these devices explain the numerically observed formation of single and multiple copies of the input beam. As the analogous devices constructed in a standard planar geometry, the proposed PC devices can be employed as filters and demultiplexers. The use of PC structures, where the light is truly confined by the stopband, adds the valuable benefit of a dramatic reduction of the overall geometrical dimensions