It is shown that the dynamic elastic-wave equations can be integrated quite simply in the neighborhood of the tip of a moving crack. A simple approximation to the exact solution is given. From examination of the dynamic stress field, arguments are presented to explain the increase in fracture toughness and fracture surface roughness with crack velocity. It is also postulated that fracture paths, asymmetrical to the applied loading, can be stable if their velocity of crack propagation is great enough.