Rapidly increasing fuel prices and global pressures to reduce the harmful effects of vehicle emissions are forcing vehicle manufacturers to look at alternative materials to reduce the mass of the vehicles they produce. This has led to an increase in the research and development of magnesium alloys for use in automotive applications, and the methods used to produce them. With a density two-thirds that of aluminium alloys, and one quarter that of steel, magnesium alloys are continually being investigated as possible replacements for the production of automotive castings such as wheels and engine blocks. However, there is presently a lack of understanding of the effects of common alloy additions that refine the as-cast microstructure on the castability of the casting alloys in question. The most popular magnesium alloys for large automotive castings are based on the magnesium-aluminium alloy system. For the work presented in this thesis, two of the most common Mg-Al alloys, AZ91E and AM60B, were chosen to determine the effect of grain refinement on the castability of Mg-Al alloys. An experimental mould has been designed and developed to emulate the geometry, filling, and solidification of an alloy wheel casting. Two different grain refiners were investigated at varying addition levels to determine the effect of different levels of grain refinement on the castability of the two alloys. Castability, defined as the ability to produce sound castings from a particular alloy, has been assessed through visual examination of external casting defects, X-ray radiography of internal defects, as well as grain size and porosity level analysis of all samples produced. It has been found that for both AZ91E and AM60B, an increase in the level of grain refinement achieved lead to an increase in the occurrence and severity of the surface slumping defect in the castings produced. Conversely, increased levels of grain refinement were found to reduce or totally eliminate the occurrence of the hot tearing defect in both alloys. Contrary to popular belief, grain refinement was found to increase the level of internal shrinkage porosity in the Mg-Al castings. This was especially the case in the poorly fed regions of the casting, such as the junction between the spoke and rim sections. This is due to the increased reliance on interdendritic feeding of liquid to compensate for shrinkage during the latter stages of solidification in such areas, where solidification does not proceed directionally towards the feeding reservoirs. An increase in the level of grain refinement means the interdendritic network becomes heavily constricted, resulting in a rapid increase in the feeding pressure required to feed metal to the areas where it is required. This study has also resulted in the development of a novel method of successfully and consistently introducing carbon to a Mg-Al alloy melt. This has in the past been identified as a major issue to overcome when adding carbon based grain refiners to Mg-Al alloy melts.