  In the continuum limit, the theory of dislocations in crystals predicts a
divergence in the elastic energy of the host material at a crystal geometry
dependent limiting (or critical) velocity $v_c$. Explicit expressions for $v_c$
are scattered throughout the literature and are available in analytic form only
for special cases with a high degree of symmetry. The fact that in some cases
(like pure edge dislocations in fcc) $v_c$ happens to coincide with the lowest
shear wave speed of a sound wave traveling parallel to the dislocation's
gliding direction has led to further confusion in the more recent literature.
The aim of this short review therefore is to provide a concise overview of the
limiting velocities for dislocations of arbitrary character in general
anisotropic crystals, and how to efficiently compute them, either analytically
or numerically.
