This volume contains revised versions of selected papers presented during the 28th Annual Conference of the Gesellschaft fur .. Klassi?kation (GfKl), the German Classi?cation Society. The conference was held at the Universit. at Dortmund in Dortmund, Germany, in March 2004. Wolfgang Gaul chaired the program committee, Claus Weihs and Ernst-Erich Doberkat were the local organizers. Patrick Groenen, Iven van Mechelen, and their colleagues of the Vereniging voor Ordinatie en Classi?catie (VOC), the Dutch-Flemish Classi?cation Society, organized special VOC sessions. The programcommittee recruited17 notable and internationallyreno- ed invited speakers for plenary and semi-plenary talks on their current - search work regarding classi?cation and data analysis methods as well as - plications.Inaddition,172invitedandcontributedpapersbyauthorsfrom18 countrieswerepresentedatthe conferencein 52parallelsessionsrepresenting the whole ?eld addressed by the title of the conference "Classi?cation: The Ubiquitous Challenge". Among these 52 sessions the VOC organizedsessions on Mixture Modelling, Optimal Scaling, Multiway Methods, and Psychom- ricswith18papers.Overall,theconference,whichistraditionallydesignedas an interdisciplinary event, again provided an attractive forum for discussions and mutual exchange of knowledge. Besides the results obtained in the fundamental subjects Classi?cation and Data Analysis, the talks in the applied areas focused on various app- cation topics. Moreover, along with the conference a competition on "Social Milieus in Dortmund", co-organized by the city of Dortmund, took place. Hence the presentation of the papers in this volume is arranged in the f- lowing parts: I. (Semi-)Plenary Presentations II. Classi?cation and Data Analysis III. Applications, and IV. Contest: Social Milieus in Dortmund.