A theory of diffusion‐controlled dislocation damping is developed by considering the motion of pinning points within the dislocation core when an oscillating driving stress is applied to a crystal. At low temperatures, the pinning points are immobile and the result is equivalent to that of the Granato‐Lu¨cke theory of resonant dislocation damping. At higher temperatures, however, the pinning points migrate under the influence of an externally applied stress &sgr;, if &sgr; exceeds a critical value &sgr;c, and the resonant‐type loss becomes time dependent. When &sgr; is reduced below &sgr;c, the pinning points thermally diffuse to their original equilibrium positions and the time‐dependent damping, characterized by a relaxation time &tgr;(0), nearly decays exponentially. In addition, a relaxation‐type loss is predicted when the applied frequency approximately equals &tgr;−1(0). The analysis relates &tgr;(0) to the ``pipe'' diffusion coefficient of the pinning speciesDpby the expression &tgr;(0)=2l02/Dp, wherel0is the average dislocation loop length, and procedures are suggested that allow the determination ofDpfrom dislocation damping measurements.