We investigate the ageing kinetics of a ternary Al-Zn-Mg alloy, and the coupling between prestraining and subsequent ageing. The precipitation sequence is investigated using DSC. The microstructure is followed using TEM and Small-Angle X-ray Scattering, and the hardening resulting from precipitation studied by hardness measurements. The effect of prestraining depends significantly on the parameters of the thermomechanical treatment, and has to be considered in terms of competition between homogeneous precipitation and heterogeneous precipitation on dislocations. The effect of prestraining on peak hardening is controlled by the nucleation of the hardening η′ phase, and therefore depends critically on the heating rate to the ageing temperature. The effect of prestraining on overageing is an increase in overall coarsening kinetics due to the combination of faster precipitate coarsening on dislocations and dissolution of homogeneous precipitates followed by solute diffusion towards dislocations.