Metal-organic frameworks are an exciting class of molecularly precise, nanoporous materials. Because of their high level of molecular precision, they can be tailored to specific applications, giving promise to a world of “crystal engineering” in which porous materials are rationally designed at the molecular level. Indeed, metal-organic frameworks have shown considerable potential in a wide variety of applications, including gas storage and separations, targeted drug delivery, and catalysis. Despite their potential, a very limited number of metal-organic frameworks have been successfully commercialized, in large part due to challenges in their synthesis. One of the major challenges in the synthesis of metal-organic frameworks is controlling the crystal phase of the material; that is, how the atoms are arranged within the framework. My research seeks to understand the factors that govern crystal phase-selection in the formation of metal-organic frameworks and to use these insights to develop new methods of controlling the phase. These studies will enable the rational design of new materials.