Experimental data on the plastic deformation of icosahedral Al-Pd-Mn single quasicrystals are discussed in terms of dislocation models. The occurrence of a yield drop followed by a range of steady state deformation is explained by the effects of dislocation multiplication and recovery on both the mobile dislocation density and the internal stress. The influence of transient phenomena during the measuring procedures of the activation parameters is estimated. The activation volume of deformation is a unique function of the stress. The activation enthalpy and the Gibbs free energy, calculated by Schoeck's formula, assume approximately constant values above 700 °C but decrease strongly below this temperature. In the temperature range where steady state deformation is possible, probably the flow stress is controlled by both the friction mechanism of moving dislocations and the recovery processes taking place during deformation.