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Recent experimental observations of intersonic shear crack propagation,occurring in a variety of material systems, have rekindled interest in the study of the intersonicfailure phenomenon. Since the early 90s, engineers and scientists working in all length scales,from the atomistic, the structural, all the way up to the scale of the earth’s deformationprocesses, have joint efforts to study this unexplored area of fracture mechanics. The structureof the analysis presented in the article emphasizes the cooperative and complementary mannerby which the experimental observations and the analytical and numerical developments havecomplimented each other. The article first reviews early contributions to the theoreticalliterature of dynamic subsonic and intersonic fracture and highlights the significant differencesbetween tensile and shear cracks. The article then uses direct laboratory observations as aframework for discussing the physics and the mechanics of intersonic shear crack propagationoccurring in constitutively homogeneous (isotropic and anisotropic), as well as ininhomogeneous systems, all containing preferable crack paths. Experiments and models areused to discuss processes such as (1) shear shock wave formation, (2) large-scale frictionalcontact and sliding at the crack faces, and (3) maximum attainable crack speeds and crackspeed stability.