Effective grain refinement strongly depends on the interplay between nucleation potency of nucleant particles, grain initiation behaviour and solidification conditions. Recently, we have made good progress in understanding grain initiation behaviour, and developed the concepts of progressive grain initiation and explosive grain initiation. We found that the most effective grain refinement can be achieved by impeding nucleation using nucleant particles of least potency, which is in direct contrast to the traditional approach to grain refinement, in which the most potent nucleant particles are used to enhance heterogeneous nucleation. In this contribution we present our latest understanding of effective grain refinement though analytical and numerical modelling of solidification processes. We will show the grain initiation behaviour can be best presented by a grain initiation map, which can be used as a practical guide for effective grain refinement.