This paper will use a new method for predicting grain size and then apply it to various solidification environments to reveal which factors are dominant in determining the final grain size. This study will only focus on methods where the grain size is set during a solidification process. These processes include grain refinement by inoculation of the melt with grain refining particles, increasing the cooling rate, low superheat casting, ultrasonic treatment and the use of chill moulds. Each of these methods can control the grain size to some extent but in order to predict the outcome it needs to be understood how the alloy constitution, inoculant particle characteristics and the casting conditions affect the prime nucleation event for the formation of new grains. These methodologies are currently being used, or have potential to be used, for the production of fine spherical grained semi-solid slurries.