The integration of sample pretreatment into microfluidic devices represents one of the remaining hurdles towards achieving true miniaturized total analysis systems (μTAS). The challenge is made more complex by the enormous variation in samples to be analyzed. Moreover, the pretreatment technique has to be compatible with the analysis device to which it is coupled in terms of time, reagent and power consumption, as well as sample volume. This review provides a thorough overview of the developments in this field to date. © 2002 Elsevier Science B.V. All rights reserved.