Abstract : The propagation of elastic waves in infinite periodic structures is described by the Bloch theory. Due to experimental constraints the phononic crystals are necessarily of finite size; some discrepancies between the theory and the experimental data are thus possible. Moreover, the behaviour of elastic waves within these periodic structures is badly described thus far, because it is deduced from the transmitted signal. Therefore, a large number of phenomena, existing only within crystals, cannot be brought to light. The first part of this experimental work aims at better understanding the dispersion of elastic waves propagating within bi-dimensional phononic crystals. These phononic crystals consist of air inclusions engraved in a thin silicon plate by photolithography and chemical attack. A laser-ultrasonic setup is used both to generate and to detect surface elastic waves on all the surface of the sample. The analysis of the displacements field at the surface of the samples allows reveals the decomposition of wave vectors, as predicted by the Bloch theory. At the band gap edge, an observation of elastic modes with zero group velocity has also been achieved. Finally, the influence of the dissymmetry of the inclusions on the opening of intra-band gap has been studied. The second part of this work is devoted to the negative refraction of Lamb and Rayleigh waves by two dimensional phononic crystals with a solid matrix (silicon and silica). The link between the propagation of elastic waves within the phononic crystal and the refraction at the interface with the homogeneous medium is established.