The collision of plates at high velocity sometimes causes them to be metallurgically bonded. A study has been made of the nature of the bonds and of the conditions required to produce bonding. A necessary requirement is the formation of a jet in the space ahead of the region of impact. When the collision region moves along the plates at a velocity which exceeds the bulk sound velocity of both materials, the relative velocity of the plates must exceed a critical value for jetting. Clarification of these critical conditions in symmetric collisions, and a correct extension to asymmetric collisions have been obtained by consideration of the effect of downstream boundary conditions on the configuration of the shock waves attached to the collision line in the jetless case. When the velocity of the collision is subsonic, bonding is obtained when the elastic strength of the material is exceeded. Metallographic examination shows that three types of bond may be formed: (1) a direct bond, (2) a uniform layer of solidified melt, and (3) a wavy interface with discrete regions of solidified melt alternating with regions of direct bond. Consideration of the flow indicates the following: (1) the direct bond results from the removal of the surface layers as part of the jet, (2) the alloy results from the melting caused by entrapment of the high‐velocity jet between the plates, and (3) the wavy interface is caused by an oscillation in the jet flow which produces the mixed type of bonding. The thickness of the continuous alloy layer, which greatly exceeds the calculated jet thickness, indicates that entrapment of the jet dissipates most of the kinetic energy lost in the collision. Analysis of the layer indicates a composition and structure which is expected from the rapid quenching of a well‐mixed melt.