Logics in Artificial Intelligence : 9th European Conference JELIA 2004 Lisbon Portugal September 27 30 2004. Proceedings This book constitutes the refereed proceedings of the 9th European Conference on Logics in Artificial Intelligence JELIA 2004 held in Lisbon Portugal in September 2004. The 52 revised full papers and 15 revised systems presentation papers presented together with the abstracts of 3 invited talks were carefully reviewed and selected from a total of 169 submissions. The papers are organized in topical sections on multi agent systems; logic programming and nonmonotonic reasoning; reasoning under uncertainty; logic programming; actions and causation; complexity; description logics; belief revision; modal spatial and temporal logics; theorem proving; and applications