Thin wall casting of aluminum alloys and their composites provide new opportunities for aerospace and automotive industries in producing light weight structures with good mechanical properties. These opportunities can be realized only when the foundries are confident in their ability to produce thinner-wall castings. The primary issue that must be addressed in producing thin wall castings is fluidity. The fluidity of aluminum alloys and composites has a direct influence not only on material castability, but also on the casting properties. Fluidity is a complex parameter that is affected by the properties of the molten metal and mold, pouring conditions, reinforcement characteristics and solidification mechanism. This paper brings out the current understanding about the various factors influencing the fluidity of aluminum alloys and composites. Further, theoretical model available in the literature to predict the fluidity of aluminum alloys and their composites is given and its significance has been discussed.