Heterogeneous nucleation and grain initiation are two different processes in early-stage solidification (ESS), although both are deterministic. Heterogeneous nucleation refers to the formation of a 2-dimensional (2D) nucleus (a crystal plane of the solid) that can template further growth, while grain initiation is the formation of a hemispherical cap (3D) from which isothermal growth is possible. It is both theoretically and practically beneficial to separate heterogeneous nucleation from grain initiation. This paper provides an overview of our recent understanding of grain initiation behaviour under different conditions and its consequences on grain refinement. After a brief review of the processes involved in the ESS, we present the grain initiation behaviour on a single substrate. This is followed by grain initiation behaviour in systems with a population of nucleant particles with varying particle types (corresponding to varying nucleation undercoolings), where we give detailed descriptions of progressive grain initiation, explosive grain initiation, hybrid grain initiation, grain initiation maps and grain refinement maps. We then provide a brief introduction to the rules that govern competition for heterogeneous nucleation and grain initiation among multiple types of nucleant particles with varying particles’ nucleation undercoolings and sizes. Finally, we present the practical implications of grain refinement maps to grain refinement. A key finding from this work is that more significant grain refinement can be achieved by promoting explosive grain initiation using impotent nucleant particles, which is opposite to the traditional approach for grain refinement where potent particles are used to enhance heterogeneous nucleation.