Master alloys are routinely added into aluminum melts before casting for grain refinement purposes. The widely used master alloys contain titanium and boron in the forms of Al3Ti and TiB2 particles in an aluminum matrix. Upon addition into aluminum melts, Al3Ti dissolves into the aluminum melts and promotes the heterogeneous nucleation of the α-Al grains while restricting the growth of α-Al grains through a constitutional cooling effect in solidification. Meanwhile, TiB2 is stable and acts as a substrate for the heterogeneous nucleation of α-Al grains through a layer of Al3Ti on the surface. Sharing these in common, different mechanisms for the grain refinement of aluminum by Al-Ti-B-type master alloys have been proposed. Another kind of popular master alloys is Al-Ti-C, which is used in a lesser extent for the grain refinement of Al alloys containing elements that are poisoning Al-Ti-B master alloys. Titanium and carbon exist as Al3Ti and TiC particles. TiC is not as stable as TiB2 and decomposes in aluminum melts. TiC in Al-Ti-C therefore acts as heterogeneous nucleation sites for α-Al grains similar to TiB2. However, the fading of Al-Ti-C master alloys is irreversible, which is the major disadvantage of Al-Ti-C master alloys. Al-Ti master alloys do not contain hard particles and are used industrially for products that are sensitive to hard TiB2 and TiC particles. There are also other master alloys that show high grain refinement potentials in lab tests but have never been used industrially for mainly low-volume production and high costs. This article gives an overview of the grain refinement of aluminum by master alloys with an emphasis on Al-Ti-B master alloys, from the production to the industrial applications of the master alloys.