The experimental aspects of the plastic deformation of icosahedral quasicrystals are reviewed. Macroscopic experiments, which involve the general investigation of stress-strain curves and the determination of the thermodynamic activation parameters of the deformation process are described. Investigations of the microstructure of plastically deformed samples, studied by means of transmission electron microscopy are presented. Important parameters such as the dislocation density, the Burgers vectors of dislocations, and slip systems are analyzed. Additionally, the results of in-situ straining experiments giving direct insight into the dynamics of the deformation process are presented. Direct conclusions on the nature of the plastic deformation process are drawn, and the current view of the deformation mechanism based on the specific structure of this class of materials is consistently discussed in terms of a qualitative 'cluster friction model'.