Topological defects, such as domain walls and vortices, have long fascinated physicists. A novel twist is added in quantum systems like superfluid helium He$_3$, where vortices are associated with low energy excitations in the cores. Similarly, cosmic strings which are vortices of the Higgs field, may be tied to propagating fermion modes. Can analogous phenomena occur in crystalline solids that host a plethora of topological defects? In this talk I will show that indeed dislocation lines are associated with one dimensional fermionic excitations in a `topological insulator', a novel band insulator believed to be realized in the bulk material Bi$_{0.9}$Sb$_{0.1}$. In contrast to electrons in a regular quantum wire, these modes are topologically protected, and not scattered by disorder. Since dislocations are ubiquitous in real materials, these excitations could dominate spin and charge transport in topological insulators. Our results provide a novel route to creating a potentially ideal quantum wire in a bulk solid.