  We consider the visco-elastic response of the electronic degrees of freedom
in 2D and 3D topological insulators (TI). Our primary focus is on the 2D Chern
insulator which exhibits a bulk dissipationless viscosity analogous to the
quantum Hall viscosity predicted in integer and fractional quantum Hall states.
We show that the dissipationless viscosity is the response of a TI to torsional
deformations of the underlying lattice geometry. The visco-elastic response
also indicates that crystal dislocations in Chern insulators will carry
momentum density. We briefly discuss generalizations to 3D which imply that
time-reversal invariant TI's will exhibit a quantum Hall viscosity on their
surfaces.
