Abstract: Methods of heterogeneous crystallization and related systems are provided. In some embodiments, a method comprises crystallizing an agent in a suspension comprising a heteronucleant and the dissolved agent. Crystallization may occur on the surface of the heteronucleant with little or no bulk crystallization and/or secondary nucleation. In some embodiments, a crystallizer may be configured to inhibit secondary nucleation and/or bulk crystallization, for example, by reducing the formation of free crystals that may serve as nucleation surfaces while allowing for adequate mass and heat transfer. In some such embodiments, the crystallizer may be designed to flow (e.g., continuously) a suspension comprising a heteronucleant and an agent in a fluidized state. The methods and systems of the present invention may be used in a wide variety of applications, including the crystallization of pharmaceutically active agents.