Preconcentration is a very important issue for achievement of low detection limits. Various on-line preconcentration modes, various electrolyte compositions, pH values and mechanisms responsible for the focusing effect are described in literature. The main modes of stacking, sweeping and transient isotachophoresis are discussed. Some of the described methods are combinations of major techniques of preconcentration and less known focusing techniques. References for optimization of focusing conditions and routine use of on-line preconcentration are given.