The propagation of a pre-existing edge crack across a finite plate subjected to cold shock has been studied. The plate, initially at uniform temperature, is exposed to a cold shock on one surface whilst three dierent types of heat transfer boundary condition are separately considered for the opposing face: cold shock, thermal insulation and fixed temperature. For all three boundary conditions, the plate experiences tensile stress near the cold-shocked surface and compressive stressing near the mid-plane. Consequently, a Mode I edge crack extending into the compressive region may grow in one of three different modes: continued extension in plane strain, channelling and spalling. The thermal shock conditions governing each failure mode are quantified, with a focus on crack channelling and spalling. The dislocation method is employed to calculate the energy release rates for plane strain cracking and steady-state channelling. For steady-state spalling, the energy release rate is obtained by an energy analysis of elastic beams far ahead and far behind the crack tip. Analytical solutions are also obtained in the short crack limit in which the problem is reduced to an edge crack extending in a half space; and the parameter range over which the short crack solution is valid for a finite plate is determined. Failure maps for the various cracking patterns are constructed in terms of the critical temperature jump and Biot number, and merit indices are identified for materials selection against failure by thermal shock.