In this final thesis differently aged specimens of aluminium alloy 7075 were water peened and their mechanical properties were analised. The influence of water peening on surface roughness, electrical conductivity, microhardness and microstructure of aged aluminium alloy was investigated. First the aluminium alloy was aged at 145°C, 170°C or 195°C, besides one specimen that was not aged. All of the specimen were water peened with 40% coverage. Analysis of the results shows that the surface roughness and hardness of material increase after water peening. Overaging of the material results in higher surface rougness and barely any change in hardness. Water peening of the overaged material can even result in decreased hardness of the material.