This article summarizes the metallurgy of carbon and alloy steels, followed by discussions on their major mechanical properties, namely, static fracture toughness, dynamic fracture toughness, fatigue or sustained-load crack growth rates, and fatigue or sustained-load thresholds. It addresses fatigue crack propagation and sustained-load crack propagation, as well as the fundamental aspects of fracture in steels. The article illustrates the effects of variations in the alloy chemistry, microstructure, temperature, strain rate, and environment on various fracture toughness or crack growth rate parameters.