The development of ionic liquids with low viscosity and high ionic conductivity is important for many applications of ionic liquids, such as their use as electrolytes for electronic devices. The transport properties depend on the motion of ions in ionic liquids. The viscosity becomes low and the ionic conductivity becomes high as the diffusion of ions becomes fast. Therefore the elucidation of the factors controlling the diffusion of ions is needed for developing new ionic liquids. In this Minireview we discuss the factors controlling the diffusion of ions in ionic liquids (size of ion; shape of ion; magnitude of interaction between cation and anion; conformational flexibility; molecular mass of ion; mixture of solute; and the effects of nanostructures in ionic liquids). We emphasize the importance of the use of computational methods (molecular dynamic simulations and quantum mechanical calculations) combined with experimental measurements for elucidating the factors controlling the diffusion of ions in ionic liquids.