ConspectusThe synthesis of monodisperse colloidal nanomaterials with well-defined structures is important for both fundamental research and practical application. To achieve it, wet-chemical methods with the usage of various ligands have been extensively explored to finely control the structure of nanomaterials. During the synthesis, ligands cap the surface and thus modulate the size, shape, and stability of nanomaterials in solvents. Besides these widely investigated roles of ligands, it has been recently discovered that ligands can affect the phase of nanomaterials, i.e., their atomic arrangement, providing an effective strategy to realize the phase engineering of nanomaterials (PEN) by selecting appropriate ligands. Nanomaterials normally exist in the phases that are thermodynamically stable in their bulk states. Previous studies have shown that under high temperature or high pressure, nanomaterials can exist in unconventional phases which are unattainable in the bulks. Importantly, nanomaterials with unconventional phases exhibit unique properties and functions different from conventional-phase ones. Consequently, it is feasible to utilize the PEN to tune the physicochemical properties and application performance of nanomaterials. During wet-chemical synthesis, ligands binding to the surface of nanomaterials can modify their surface energy, which could significantly affect the Gibbs free energy of nanomaterials and thus determine the stability of different phases, making it possible to obtain nanomaterials with unconventional phases at mild reaction conditions. For instance, a series of Au nanomaterials with unconventional hexagonal phases have been prepared with the assistance of oleylamine. Therefore, the rational design and selection of different ligands and deep understanding of their effect on the phase of nanomaterials would significantly accelerate the development of PEN and the discovery of novel functional nanomaterials for diverse applications.In this Account, we briefly summarize the recent progress in ligand-assisted PEN, elaborating the important roles of different ligands in the direct synthesis of nanomaterials with unconventional crystal phases and amorphous phase as well as the phase transformation of nanomaterials. We first introduce the background of this research topic, highlighting the concept of PEN and why ligands can modulate the phase of nanomaterials. Then we discuss the usage of four kinds of ligands, i.e., amines, fatty acids, sulfur-containing ligands, and phosphorus-containing ligands, in phase engineering of different nanomaterials, especially metal, metal chalcogenide, and metal oxide nanomaterials. Finally, we provide our personal views of the challenges and future promising research directions in this exciting field.