Griffith's 1921 theory of crack stability is extended to account for the frictional energy dissipation associated with interfacial sliding when the crack faces are in contact. This global energy balance approach is essentially different from Griffith's 1924 local theory of compression cracks, which assumes that instability occurs when the stress at any point exceeds the intrinsic strength of the material. An explicit expression for critical crack size in compression is obtained. On the basis of this expression we find the most critical crack orientation and define theoretical strength as the critical stress for that orientation. This uniaxial compression strength significantly exceeds the tensile strength. For example, when the coefficient of friction is unity, the ratio of compressive to tensile strength is 8.7, and the crack plane lies at 13¿ to the loading direction. Following this, the heating due to interfacial sliding is found by a one-dimensional thermoelastic analysis. It is concluded that under normal conditions the stress field acting on the crack faces is only slightly affected by frictional heating, though the temperature rise may be very high, and melting may occur within microseconds.