  We discuss the formation of a vortex polycrystal in type II superconductors
from the competition between pinning and elastic forces. We compute the elastic
energy of a deformed grain boundary, that is strongly non-local, and obtain the
depinning stress for weak and strong pinning. Our estimates for the grain size
dependence on the magnetic field strength are in good agreement with previous
experiments on NbMo. Finally, we discuss the effect of thermal noise on grain
growth.
