This chapter describes how to obtain by experiments the material toughness, the energy absorbed during the Charpy impact tests, the transition temperature concept, and presents the master curve approach. Besides these basic points, the neutron irradiation is a concern in the nuclear industry because it influences the material properties, notably reduces its toughness. Due to this fact, it is necessary to implement the so-called material or plant surveillance program that monitors the material properties evolution with the time and neutron fluence. All this is conducted to protect the reactor vessel against brittle fracture.