This paper overviews the physical processes that mediate cleavage fracture toughness from atomic to structural length scales. We first describe the toughnesstemperature master curve shift method to assess embrittlement following neutron irradiation of structural steels. Based upon a well-calibrated local approach of cleavage, we present an optimized adjustment procedure to transfer fracture toughness data from one specimen geometry to another one. The adjustment procedure is illustrated by two large databases: one for a reactor pressure vessel steel and one for an advanced high-chromium steel. The observed constant shape of the master-curve after irradiation is believed to result from a moderate temperature dependence of the critical local microcleavage stress. We briefly summarize the results of a recent experimental study of the temperature dependent initiation (KIc) and arrest toughness (Ka) of cleavage oriented (100)[010] and (100)[011] Fe single crystals that support this hypothesis.