This volume contains the papers presented at the 3rd International Symposium onFoundationsofInformationandKnowledgeSystems(FoIKS2004), whichwas held in Castle Wilhelminenberg, Vienna, Austria, from February 17th to 20th, 2004. FoIKS is a biennial event focussing on theoretical foundations of information and knowledge systems. It aims at bringing together researchers working on the theoretical foundations of information and knowledge systems and attracting researchers working in mathematical ?elds such as discrete mathematics, c- binatorics, logics, and ?nite model theory who are interested in applying their theories to research on database and knowledge base theory. FoIKS took up the tradition of the conference series Mathematical Fun- mentals of Database Systems (MFDBS) which enabled East-West collaboration in the ?eld of database theory. The ?rst FoIKS symposium was held in Burg, Spreewald (Germany) in 2000, and the second FoIKS symposium was held in SalzauCastle(Germany)in2002.FormerMFDBSconferenceswereheldinDr- den (Germany) in 1987, Visegr ad (Hungary) in 1989, and in Rostock (Germany) in 1991. Proceedings of these previous events were published by Springer-Verlag as volumes 305, 364, 495, 1762, and 2284 of the LNCS series, respectively. In addition the FoIKS symposium was intended to be a forum for intensive discussions. For this reason the time slots for long and short contributions were 50 and 30 minutes, respectively, followed by 20 and 10 minutes for discussions, respectively. Furthermore, participants were asked in advance to prepare to act as correspondents for the contributions of other authors. There were also special sessions for the presentation and discussion of open research problems."