The two main types of degradation that control the lifetime of the components in a nuclear plant, notably in PWR plants, are fatigue and stress corrosion cracking (SCC). This is because most of the plant structural elements (reactor vessel, steam generator, pressurizer, turbine, pumps, and piping connecting these components) are subject to cyclic loads in combination with an aggressive environment. The fatigue failure occurs only under cyclic load while SCC can occur under a steady load. Corrosion fatigue is not approached as well as creep damage (it is not a concern in a PWR as the plants operate below 350 °C). Besides the short introduction of the history of fatigue and stress corrosion, the processes for crack initiation and propagation (growth) under these two effects are introduced, along with the crack propagations in accordance with the ASME Code Section XI.