particulate matter from a gas stream. Rational design of the filtration process should be based on reliable prediction of the effluent concentration and pressure drop for a given set of operating conditions. Moreover, both of these main filter parameters vary in time when a fibrous structure becomes more and more loaded with deposits. Usually, fibrous filters with a very high porosity can assure a long Iife-time for a given level of pressure drop and filtration efficiency and therefore the basic issue is an improvement of the clean filter characteristics, that is an increase of filtration efficiency maintaining a low pressure drop. One of the promising methods is an application of electret fibers in the filter structure. Electret fibers have a "built-in" electric charge and they preserve this charge for a very long time. Due to the electret characteristics of the fibers, additional forces of e1ectrical nature act between them and particles. The collection of submicron particles is considerably improved. Efficient and highly porous electret filters of low flow resistance are preferably used in respirators, clean room filters and ventilating systems. To stimulate an interdisciplinary discussion, Warsaw University of Technology (Prof.dr.L.Gradon) and Delft University of Technology Dr.irJ.C.M.Marijnissen) organised a Workshop on "Aerosol Filtration in Electret Fibrous Structures". The Workshop was held in the historical Jablonna Palace near Warsaw, Poland, January 28-30, 1999. This Workshop tried to bring together different disciplines to get a complete view of electret formation, nonsteady-state filtration in electret filters, filter testing and industrial application of electrets. All this information is necessary to produce optimal structures of filters. This book contains the proceedings ofthe Workshop, and it includes an additional discussion on the application ofbiodegradable polymers for fibrous filters formation.