In order to promote the development and application of chemical sensors for in vivo monitoring. a series of workshops has been organized by the European Community Concerted Action on “Chemical Sensors for Monitoring”. The third Workshop of this Concerted Action was held on 6–9 March 1990 at the University of Twente in Enschede, the Netherlands. After the lirst two orkshops, in which the clinical aspecfs (Lyon, 8–10 March 1989) and the device strategies (Florence, 12–15 November 1989) were discussed. In this workshop the main topic was the technological aspects of microsensors to be used for hyirnsensing. A report of the third workshop is presented, which is entitled: “In yirp sensing: fundamental approaches and potential promises 01 available technologies”.