The mobility of dislocations is an important factor in understanding
material strength. Dislocations experience a drag due to their
interaction with the crystal structure, the dominating contribution at
high stress and temperature being the scattering off phonons due to
phonon wind. Yet, the functional dependence of this effect on velocity
and other properties has eluded a good theoretical understanding.
In this talk we present recent results on dislocation drag from first
principles as a function of velocity (resp. stress), temperature,
density, and character. We then discuss the impact this newly derived
drag coefficient has on the overall material response by presenting
selected examples from single crystal plasticity simulations as well as
an analytical model of stress as a function of strain rate which takes
into account mobile-immobile dislocation intersections as well as
dislocation drag from phonon wind. We also comment on the possibility
of supersonic dislocations.