During the last decade, a great interest in the preparation of uniform nanocrystals has offered efficient synthetic strategies to precisely engineer metal, metal oxide and alloy at the nanoscale. Due to their physicochemical properties, ionic liquids (ILs) simultaneously demonstrated their potential in different areas such as nanocrystal synthesis, catalysis and energy. As a result, ionic liquids have been employed for the preparation of monodisperse nanocrystals and the control of their size. This chapter highlights the most promising methods for the synthesis of uniform nanocrystals in ionic liquids which act as a solvent, stabiliser, reducing agent and even precursor. As a result, successful preparations of nanoparticles in the presence of ILs are now available for both noble and earth-abundant elements such as gold, platinum, iridium, silver, palladium, ruthenium, rhodium, copper, nickel, cobalt and iron. The formation mechanisms of these nanocrystals are discussed as well as our mechanistic understanding in conventional organic and aqueous solvents. In addition, the IL approach is compared to leading methods in conventional solvents to make possible the identification of general principles for most metallic elements. By analogy with conventional solvents, these strategies can be adapted to the preparation of semiconductor nanocrystals. These achievements are going to drive the identification of relationships between the nature of ILs components, the physicochemical properties of ILs, the formation of nanocrystals in ILs and the resulting performances of these nano-objects.