J. Prada,​Soils and fractured rocks are particulate materials, i.e., their strength, stiffness and dilative/contractive behavior are controlled by the state of effective stress; in addition, fine grained particles are also affected by contact-level electrical and capillary forces. Elastic waves propagate through these materials assessing their state, without altering their properties or processes taking place within them. Furthermore, wave-based boundary measurements can be inverted to render a tomographic image of the medium. Therefore, the propagation of elastic waves opens unique possibilities for powerful research and engineering tools in geo-materials. This paper reviews the most relevant aspects of wave propagation in geo-materials, and presents two case studies in which tomographic images are obtained. The interpretation of tomographic images in terms of the stress field is discussed.