Previous explanations of strengthening of glass‐ceramics after heat treatment to produce crystallization are discussed. Explanations relating strength enhancement in brittle material systems either to flaw size limitation by interparticle spacing or to the increase in the system modulus as a result of the presence of a higher modulus dispersed phase were found to be inadequate. The measured loss of strength in glass‐ceramics with increasing temperature indicated strength enhancement as a result of thermomechanically induced stresses in the sys