The strength of the lithosphere can be defined as the vertical integration, from the top to the bottom of the lithosphere, of the differential stress required to trigger either brittle failure or the flow failure of rocks. Failure can occur by power-law creep or Dorn law creep at high temperature and low strain-rate, or by frictional sliding at low temperature and high strain rate, in which case the differential stress depends of the tectonic regime. However at any given depth, the failure mechanism is the one that requires the minimum differential stress to operate.