Spallation occurs when two rarefaction waves interact in a body and produce enough tension to fracture it. The principal content of investigations of spall phenomena that have been conducted are: (i) metallographic examination of the spall zone in recovered samples to obtain the correlation of the degree of damage with various features of the stress history [1-3] and (ii) instrumental measurement of resistance to dynamic fracture [4][5][6]. In the former case the result is information about the ability of the material to withstand shock loading, about the mechanism of nucleation and development of micro-cracks or pores, and statistical description of these processes. Dynamic measurements (free-surface velocity profiles or pressure on the interface with a soft material) during the shock-wave loading give the most correct data about stresses present during the spall process. The theoretical background of these measurements is discussed in Section 1.2 of this chapter.The damage rate, which is approximately equal to the product of concentration of damage nucleation sites and the average growth rate of damage at the sites, cannot be arbitrarily large. Under conditions of rapid dynamic loading, fracture is an essentially rate-dependent process. As a result, the spall strength value is not a material constant. It is more correct to speak about the resistance to fracture as a function of strain rate and other parameters of state. Since the fracture evolves over a period of time comparable with the duration of a shock wave experiment, we have to identify the stage of fracture which corresponds to the fracturing stress as this stress is determined by dynamic measurements. The acoustic analysis of spallation in relaxing media presented in Section 1.3 shows a correlation among the initial damage rate, the unloading rate in the incident shock pulse, and the freesurface velocity profile that is measured. Using this analysis, the initial damage rate can be estimated directly from the free-surface velocity profile.The typical behavior of materials of different classes-metals and alloys, brittle single crystals and glasses, and elastomers-is described in Section 1.4. A series of additional factors can influence the fracture process that