Predicting the collective effects of solid solution alloying, precipitation, and grain size on the mechanical properties of a given alloy is one of the “holy grails” of mechanical metallurgy. For alloys with a cubic crystal structure, this is challenging enough to have been solved only in specific cases, and even there, predicting the anisotropies induced by crystallographic texture is very difficult. In the case of hexagonal close packed Mg alloys, this challenge is even greater and arguably more important. Research over the past decade has also highlighted the significant impact precipitate shape and orientation can have on the strength of Mg alloys. This lecture will show, with examples from conventional and rare earth containing Mg alloys, that the crystal plasticity approach has enabled the level of our predictive capability to rapidly approach the level, which exists in other more mature alloys systems.