This chapter provides an in-depth analysis of grain refinement techniques for aluminum alloys, which are extensively used in the aerospace industry due to their unique combination of properties. The process of grain refinement, which involves decreasing the size of grains in a metallic material to create a more uniform and fine-grained microstructure, is crucial for enhancing the mechanical properties of aluminum alloys. There are several mechanisms at play in the grain refinement of aluminum alloys, including those that take place during solidification and those that occur during subsequent thermomechanical and plastic processing. The chapter discusses a variety of techniques such as chemical, thermal, mechanical, and plastic deformation methods. The effect of these methods on the results of grain refinement, the mechanisms of these methods, and their practical applications have been discussed in detail. Future research directions on grain refinement of aluminum alloys are also mentioned.