In this paper, we review progress on understanding the role of threading dislocations (TDs) in the physical properties of GaN and its alloys. A growing body of work provides compelling evidence that TDs in the group-III nitrides behave as non-radiative recombination centers, have energy levels in the otherwise forbidden energy gap, act as charged scattering centers in doped materials, and provide a leakage current pathway. In comparison with conventional III-V semiconductors, the relatively small minority carrier diffusion length L(d), (similar to 50 nm) in the III-nitrides, combined with favorable TD geometries, minimize dislocation-related degradation. The small value of L(d) also allows for appreciable optical emission in materials with TD densities as high as 10(10) cm(2).