A conceptual approach to evaluate glass-forming ability for various glass-forming systems has been proposed from a physical metallurgy point of view. It was found that the glass-forming ability for noncrystalline materials was related mainly to two factors, i.e., 1/(Tg + Tl) and Tx (wherein Tx is the onset crystallization temperature, T g the glass transition temperature, and Tl the liquidus temperature), and could be predicated by a unified parameter γ defined as Tx/(Tg +Tl). This approach was confirmed and validated by experimental data in various glass-forming systems including oxide glasses, cryoprotectants, and metallic glasses.