The use of isotachophoresis, the latest of the electrophoretic separation techniques, has grown very rapidly during the last few years in various application areas. The reason for the increased acceptance of analytical isotachophoresis is the appreciation of its attributes, such as small sample requirements in the picomole range, short analysis time of about 10 to 30 min, and ease of quantification. The present status of capillary isotachophoresis is described in this review article. A historical review is given in the first section followed by a discussion of the basic theory. Further on the selection of electrolyte systems and spacer ions is discussed, as well as aspects of quantification and different detection systems. The emphasis has, however, been laid on the description of various application areas, most of them illustrated by several characteristic practical examples of separations.