Wall cooling effectiveness is investigated for tangential, inclined, and normal injection of coolant through single or multiple wall slots into a laminar compressible boundary layer. Numerical solutions of the boundary layer equations are obtained by a reliable finite difference method. A grid control procedure which maintains a constant flow rate between grid lines is applied to the injection calculations wherein the boundary layer growth in the slot is as much as hundred-fold and the longitudinal component of the injection velocity is in some cases as large as the free stream velocity. Film cooling effectiveness is reported for a variety of injection configurations so that the effects of injection angle, coolant mass flow rate, Mach number, upstream boundary layer thickness, slot width, and the presence of upstream cooling slots can be investigated.