Abstract A fracture mechanics framework has been developed for predicting crack initiation and growth in full-scale components and structures from test specimen data. Much knowledge has also been gained about the mechanisms by which fracture occurs in a variety of materials. However, the development of quantitative connections between models of the physical processes of fracture and macroscale measures of fracture resistance is still at an early stage. A key difficulty is that fracture spans several length scales from the atomistic to the macroscopic scale. In this paper, some analyses are reviewed that use micromechanical modeling to predict fracture toughness from the physics of separation and plastic flow processes. Attention is confined to fracture by cleavage in metal crystals, under both monotonic and cyclic loading conditions. The role of models at the dislocation size scale in bridging the gap between atomistic and continuum descriptions is highlighted.