Bulk metallic glass-forming liquids are alloys with typically three to five metallic components that have a large atomic-size mismatch and a composition close to a deep eutectic. They are dense liquids with small free volumes and viscosities that are several orders of magnitude higher than in pure metals or previously known alloys. In addition, these melts are energetically closer to the crystalline state than other metallic melts due to their high packing density in conjunction with a tendency to develop short-range order. These factors lead to slow crystallization kinetics and high glass-forming ability. Crystallization kinetics is very complex, especially in the vicinity of the glass transition, due to the influence of phase separation and the decoupling of the diffusion constants of the different species.