This monograph deals with crack motion in solids under load with no acceleration term in the equations of motion. This is qualified as steadily propagating cracks. A useful quantity is G, the crack extension force per unit length of the crack front. Physical quantities associated with the crack, measured under the conditions < G > (G, averaged over the length of the crack front) positive maxima, conform to predictions. This monograph tells how the evolution of fracture in loaded solids is described.