In Al-Cu-Mg with compositions in the α+S phase field, precipitation hardening is a twostage process. Experimental evidence shows that the main precipitation sequence in alloys with Cu contents in excess of 1wt% is involves Cu-Mg co-clusters, GPBII/S'' and S. The first stage of the age hardening is due to the formation of Cu-Mg co-clusters, and the hardening can be modelled well by a modulus hardening mechanism. The appearance of the orthorhombic GPBII/S'' does not influence the hardness. The second stage of the hardening is due to the precipitation of S phase, which strengthens the alloy predominantly through the Orowan looping mechanism. These findings are incorporated into a multi-phase, multi mechanism model for yield strength of Al-Cu-Mg based alloys. The model is applied to a range of alloys with Cu:Mg ratios between 0.1 and 1 and to heat treatments ranging from room temperature ageing and artificial isothermal ageing to rapid heating to the solution treatment temperature. The predictive capabilities of this model are reviewed and its constitutive components are compared and contrasted with a range of other methods, such as the Kampmann-Wagner and JMAK models for precipitation as well as the LSW model for coarsening.