Theoretical studies of crack motion in brittle materials often use continuum models where the relation between the crack velocity v and the driving force F is found to be continuous. However, for atomistic models the relation v = v(F) is discontinuous and exhibits hysteresis. The difference is attributed to inertia, which is more important in atomistic models than in continuum models. I study the role of temperature in fracture dynamics and present a simple model study of the brittle-to-ductile transition.