Two of the most important microstructural features of alloys are the grain size and the secondary dendrite arm spacing (SDAS) and these two factors are shown to combine together to describe the grain morphology. Both grain refinement and the SDAS depend upon alloy composition through constitutional undercooling, but in different ways. It is shown that there is a ‘characteristic’ SDAS for each alloy at a particular cooling rate, and reducing the grain size causes the grain morphology to change from dendritic to cellular/rosette-like to globular or spherical. Increasing the cooling rate refines both the SDAS and the grain size, but reduces the SDAS more rapidly leading to finer, more dendritic grain structures. Particular ratios of grain size to SDAS are used to define each morphology and it is shown how these can assist with obtaining a required grain size and morphology through the use of solidification conditions and alloy chemistry.