Initially started as a minor sub-topic in the large field of Micro System Technology (MST or MEMS), the field of mTAS is currently generally considered as one of the most important application areas of MST, which is reflected in the still growing research, development, and commercialization activities. Apart from further development and refining of the research on electrophoretic separation, electrokinetically driven flow systems, cell manipulation and analysis, miniaturized flow systems and study of microfluidics, the important new area of centrifugal microfluidics on CD devices receives broad attention. On the other hand, new innovations range from topics as exotic as photoacoustic detection in microreactors and molecular emission detection on a chip to very high-pressure microreactor devices and shear-flow driven separations. This volume contains the proceedings of the fourth international symposium on Micro Total Analysis Systems (mTAS 2000), held 14-18 May 2000, at the University of Twente in Enschede, The Netherlands.