It is now well established that three different eutectic solidification mechanisms may occur in Al-Si foundry alloys. The operation of each mechanism can be controlled by altering the chemical composition and casting conditions. Recent research has focussed on the understanding of the mechanisms determining the eutectic solidification mode by investigating the growth mechanisms in ultra-high purity and commercial purity alloys, the effect of a wide range of different potential modifying elements and investigating the eutectic nucleation mode. It is concluded that nucleation of eutectic grains is prolific in unmodified commercial purity alloys. In contrast, high-purity alloys have relatively few nuclei and only a few eutectic grains nucleate. Nuclei can also be removed, or rendered inactive, by the addition of modifying elements to commercial purity alloys. There is a relationship between the number of eutectic grains and the morphology of the eutectic because the growth rate of the eutectic depends on the surface area, The three eutectic solidification modes have widely differing spatial distributions of growing eutectic and therefore significant effects on the feeding efficiency of the alloys. Porosity is significantly affected by eutectic solidification mode and there is a direct relationship between them.