Commercial hypoeutectic Al-Si foundry alloys contain between 50 and 90 vol% eutectic. Solidification of the eutectic is the last major solidification event and therefore the critical stage where casting defects, such as porosity, form in these alloys. Thermal analysis, microstructural inspection, quenching, and electron back-scatter diffraction (EBSD) mapping have been used to study the effects of a range of different modifying elements and levels on eutectic nucleation and growth. The results show that eutectic solidification can occur by three distinctly different nucleation and growth modes, in isolation or sometimes together, but controlled by the modifier element used and the concentration added. Furthermore, it is suggested that the plate-like to fibrous transition in the silicon morphology with modification is controlled by the interplay of the twin-plane re-entrant edge mechanism together with the eutectic growth rate, which is determined by eutectic nucleation and the resulting macroscopic growth mode. The eutectic nucleation mechanism and resulting macroscopic growth pattern dramatically affect the mushy zone permeability and pressure drop and therefore exert significant control on both porosity content and distribution.