Two applications of x-ray residual stress measurements in steel are reported. One application shows how residual stresses have a direct bearing on fatigue strength caused by varying material and heat treatment. The other application shows how x-ray diffraction measurements can help determine the cause of grinder cracking in carburized and hardened steel parts. Grinder cracking occurs by overheating the surface which produces tensile residual stresses that exceed the fracture strength of the steel. Also, there appears to be a relationship between retained austenite content and the grinder cracking phenomenon.