This book constitutes the refereed proceedings of the 9th European Conference on Logics in Artificial Intelligence, JELIA 2004, held in Losbon, 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 cansation; complexity; description logics; belief revision; modal, spatial, and temporal logics; theorem proving; and applications.