On-line sample preconcentration is a powerful technique to improve the detection sensitivity in terms of concentration.Several techniques have been developed for this purpose: field enhanced sample injection (FESI), transient isotachophoresis, sweeping, and dynamic pH junction.Each technique has advantages and disadvantages and hence, the most suitable technique should be employed depending on the samples and required conditions.FESI is simple and easy to implement but produces high concentration efficiency if the conditions are optimized.It is possible to selectively inject samples having a range of isoelectric points (pI) or dissociation constants (pKa) because only either positively or negatively charged analytes can be electrokinetically injected.Dynamic pH junction is based on the different principle from that of FESI but useful in concentrating acidic, basic and zwitterionic analytes because the electrophoretic velocity is depends on the pH.In this presentation three different applications of on-line sample preconcentration techniques are introduced for the analysis of peptides by CE-MS.