Mechanical, physical and chemical properties of a part depend on the size, morphology and dispersion of the constituents of the microstructure of the part's material. Therefore, this chapter discusses the diferent processes of altering the microstructure of Al-Si-based alloys to desired functional properties. These processes, commonly called modiication methods, were broadly categorised into three: chemical, thermal and mechanical methods. Chemical method, which involves the addition of some elements, in trace levels, to alloys to be modiied, is the best modiication option. The elements for modifying are called modiiers. The three commonly used modiiers (sodium, Na, strontium, Sr and antimony, Sb) are discussed. The chapter, however, notes that for optimal alloy's mechanical atributes, thermal treatment is usually combined with both chemical and mechanical modiication processes. The thermal method involves rapid cooling of alloy for modiication, while the mechanical method depends on force to break up large α-Al dendrites and plate-like Si phases.