Quenching is a critical step during age hardening.The quench sensitivity, i.e.the precipitation behaviour of Al alloys during cooling from solution annealing was extensively investigated by in-situ measurements with differential scanning calorimetry (DSC), microstructure analysis on a range of length scales and mechanical testing in the age hardened state.During quenching of high strength heat-treatable Al alloys precipitation on defects is in practice nearly unavoidable, and the quench sensitivity generally increases with increasing content of main alloying elements.