Grain refinement not only improves the mechanical performance of as-cast metallic materials but also provides an effective mechanism for controlling cast defects. Therefore, understanding the grain refinement of alloys during solidification is of both theoretical and practical significance. This work reviews the recent progress in the grain refinement  of aluminum alloys during the solidification process. New knowledge on essential steps in solidification, including  heterogeneous nucleation, grain initiation and grain growth, is introduced. Furthermore, efforts and achievements in the  development of new grain refiners and refining techniques are discussed in terms of the interface structures, particle characteristics (morphology, size and spatial distribution), and solutes, which are three key factors influencing the above  three steps.