Results from the literature and unpublished ones of the present authors are summarized which are relevant to the mechanisms governing the mobility of dislocations in icosahedral Al-Pd-Mn quasicrystals. These results concern macroscopic deformation tests, conventional transmission electron microscopy, in situ straining experiments in a transmission electron microscope, and computer simulation experiments. These experiments can best be interpreted by assuming that the dislocation motion is controlled by the thermally activated overcoming of Mackay-type clusters. The present paper gives an estimate of the activation volume of this process. It turns out that the activation volume of overcoming the clusters individually is one order of magnitude smaller than the experimentally observed one. The experimental observations can be interpreted in a consistent way in terms of the Labusch-Schwarz theory of solution hardening in crystals, which considers the interpenetrating clusters as 'extended' obs