The use of modifiers in the electrophoretic separation of multicomponent mixtures makes it possible to significantly expand the analytical capabilities of capillary electrophoresis. Modifiers of electrophoretic systems, different in nature and properties, can perform different functions: form stationary and pseudo-stationary phases, prevent the adsorption of analytes on the walls of a quartz capillary, help to reduce the limits of detection for analytes during preconcentration, etc. In this review, various versions of the application of modifiers to the electrophoretic separation and preconcentration of analytes of different nature are considered on an example of imidazolium ionic liquids, which can act as stationary and pseudo-stationary phases, and also directly participate in the online preconcentration of analytes of hydrophilic and hydrophobic nature.