The equation of motion and the stability conditions of surface cracks which are subjected to stress waves are derived from an energy balance and the law of angular momentum conservation. From the resulting differential equation the terminal crack velocity and dynamic threshold conditions can be derived. It will be shown that not only a critical stress (amplitude of the wave) but also a critical time (duration of the wave) are necessary to move the crack. These two critical quantities can be combined to a critical action. The specific action of the wave must exceed a certain minimal value for crack propagation to occur. Quantum considerations allow us to generalize this criterion further. Some simple applications of the least action law are mentioned. Supercritical stress pulses are also treated briefly. This leads to the concept of the cross section of a stress wave. Crack stability can also be treated as an eigenvalue problem.